Fix latest release can't be built

[Bitcoinera]

Closes #2

Following @sembrestels advice, this fixed it 🎉

[Bitcoinera]

By the way, I see a lot of warning in the build:

⚠️  Could not load source file "../src/index.ts" in source map of "../node_modules/ethereumjs-util/dist/index.js".
⚠️  Could not load source file "../src.ts/index.ts" in source map of "../node_modules/@ethersproject/abi/lib.esm/index.js".
⚠️  Could not load source file "../src/index.ts" in source map of "../node_modules/web3-eth-accounts/node_modules/ethereumjs-tx/dist/index.js".
⚠️  Could not load source file "../src/index.ts" in source map of "../node_modules/ethereumjs-common/dist/index.js".
⚠️  Could not load source file "../index.ts" in source map of "../node_modules/xhr2-cookies/dist/index.js".
⚠️  Could not load source file "../src/secp256k1v3-adapter.ts" in source map of "../node_modules/ethereumjs-util/dist/secp256k1v3-adapter.js".
⚠️  Could not load source file "../src/constants.ts" in source map of "../node_modules/ethereumjs-util/dist/constants.js".
⚠️  Could not load source file "../src/account.ts" in source map of "../node_modules/ethereumjs-util/dist/account.js".
⚠️  Could not load source file "../src/hash.ts" in source map of "../node_modules/ethereumjs-util/dist/hash.js".
⚠️  Could not load source file "../src/signature.ts" in source map of "../node_modules/ethereumjs-util/dist/signature.js".
⚠️  Could not load source file "../src/bytes.ts" in source map of "../node_modules/ethereumjs-util/dist/bytes.js".
⚠️  Could not load source file "../src/object.ts" in source map of "../node_modules/ethereumjs-util/dist/object.js".
⚠️  Could not load source file "../src/index.ts" in source map of "../node_modules/rlp/dist/index.js".
⚠️  Could not load source file "../src.ts/fragments.ts" in source map of "../node_modules/@ethersproject/abi/lib.esm/fragments.js".
⚠️  Could not load source file "../src.ts/abi-coder.ts" in source map of "../node_modules/@ethersproject/abi/lib.esm/abi-coder.js".
⚠️  Could not load source file "../src.ts/interface.ts" in source map of "../node_modules/@ethersproject/abi/lib.esm/interface.js".
⚠️  Could not load source file "../src/index.ts" in source map of "../node_modules/@aragon/wrapper/node_modules/ethereumjs-tx/dist/index.js".
⚠️  Could not load source file "../src/transaction.ts" in source map of "../node_modules/web3-eth-accounts/node_modules/ethereumjs-tx/dist/transaction.js".
⚠️  Could not load source file "../src/fake.ts" in source map of "../node_modules/web3-eth-accounts/node_modules/ethereumjs-tx/dist/fake.js".
⚠️  Could not load source file "../../src/chains/index.ts" in source map of "../node_modules/ethereumjs-common/dist/chains/index.js".
⚠️  Could not load source file "../../src/hardforks/index.ts" in source map of "../node_modules/ethereumjs-common/dist/hardforks/index.js".
⚠️  Could not load source file "../xml-http-request.ts" in source map of "../node_modules/xhr2-cookies/dist/xml-http-request.js".
⚠️  Could not load source file "../xml-http-request-event-target.ts" in source map of "../node_modules/xhr2-cookies/dist/xml-http-request-event-target.js".
⚠️  Could not load source file "../../src/secp256k1v3-lib/der.ts" in source map of "../node_modules/ethereumjs-util/dist/secp256k1v3-lib/der.js".
⚠️  Could not load source file "../../src/secp256k1v3-lib/index.ts" in source map of "../node_modules/ethereumjs-util/dist/secp256k1v3-lib/index.js".
⚠️  Could not load existing sourcemap of "../node_modules/@ethersproject/signing-key/lib.esm/elliptic.js".
⚠️  Could not load source file "../../src.ts/coders/address.ts" in source map of "../node_modules/@ethersproject/abi/lib.esm/coders/address.js".
⚠️  Could not load source file "../../src.ts/coders/array.ts" in source map of "../node_modules/@ethersproject/abi/lib.esm/coders/array.js".
⚠️  Could not load source file "../../src.ts/coders/boolean.ts" in source map of "../node_modules/@ethersproject/abi/lib.esm/coders/boolean.js".
⚠️  Could not load source file "../../src.ts/coders/bytes.ts" in source map of "../node_modules/@ethersproject/abi/lib.esm/coders/bytes.js".
⚠️  Could not load source file "../../src.ts/coders/fixed-bytes.ts" in source map of "../node_modules/@ethersproject/abi/lib.esm/coders/fixed-bytes.js".
⚠️  Could not load source file "../../src.ts/coders/null.ts" in source map of "../node_modules/@ethersproject/abi/lib.esm/coders/null.js".
⚠️  Could not load source file "../../src.ts/coders/number.ts" in source map of "../node_modules/@ethersproject/abi/lib.esm/coders/number.js".
⚠️  Could not load source file "../../src.ts/coders/string.ts" in source map of "../node_modules/@ethersproject/abi/lib.esm/coders/string.js".
⚠️  Could not load source file "../src/index.ts" in source map of "../node_modules/ethereumjs-util/dist/index.js".
⚠️  Could not load source file "../src.ts/index.ts" in source map of "../node_modules/@ethersproject/abi/lib.esm/index.js".
⚠️  Could not load source file "../src/index.ts" in source map of "../node_modules/web3-eth-accounts/node_modules/ethereumjs-tx/dist/index.js".
⚠️  Could not load source file "../src/index.ts" in source map of "../node_modules/ethereumjs-common/dist/index.js".
⚠️  Could not load source file "../index.ts" in source map of "../node_modules/xhr2-cookies/dist/index.js".
⚠️  Could not load source file "../src/secp256k1v3-adapter.ts" in source map of "../node_modules/ethereumjs-util/dist/secp256k1v3-adapter.js".
⚠️  Could not load source file "../src/constants.ts" in source map of "../node_modules/ethereumjs-util/dist/constants.js".
⚠️  Could not load source file "../src/account.ts" in source map of "../node_modules/ethereumjs-util/dist/account.js".
⚠️  Could not load source file "../src/signature.ts" in source map of "../node_modules/ethereumjs-util/dist/signature.js".
⚠️  Could not load source file "../src/bytes.ts" in source map of "../node_modules/ethereumjs-util/dist/bytes.js".
⚠️  Could not load source file "../src/object.ts" in source map of "../node_modules/ethereumjs-util/dist/object.js".
⚠️  Could not load source file "../src/hash.ts" in source map of "../node_modules/ethereumjs-util/dist/hash.js".
⚠️  Could not load source file "../src/index.ts" in source map of "../node_modules/rlp/dist/index.js".
⚠️  Could not load source file "../src.ts/fragments.ts" in source map of "../node_modules/@ethersproject/abi/lib.esm/fragments.js".
⚠️  Could not load source file "../src.ts/abi-coder.ts" in source map of "../node_modules/@ethersproject/abi/lib.esm/abi-coder.js".
⚠️  Could not load source file "../src.ts/interface.ts" in source map of "../node_modules/@ethersproject/abi/lib.esm/interface.js".
⚠️  Could not load source file "../src/index.ts" in source map of "../node_modules/@aragon/wrapper/node_modules/ethereumjs-tx/dist/index.js".
⚠️  Could not load source file "../src/transaction.ts" in source map of "../node_modules/web3-eth-accounts/node_modules/ethereumjs-tx/dist/transaction.js".
⚠️  Could not load source file "../src/fake.ts" in source map of "../node_modules/web3-eth-accounts/node_modules/ethereumjs-tx/dist/fake.js".
⚠️  Could not load source file "../../src/chains/index.ts" in source map of "../node_modules/ethereumjs-common/dist/chains/index.js".
⚠️  Could not load source file "../../src/hardforks/index.ts" in source map of "../node_modules/ethereumjs-common/dist/hardforks/index.js".
⚠️  Could not load source file "../xml-http-request-event-target.ts" in source map of "../node_modules/xhr2-cookies/dist/xml-http-request-event-target.js".
⚠️  Could not load source file "../xml-http-request.ts" in source map of "../node_modules/xhr2-cookies/dist/xml-http-request.js".
⚠️  Could not load source file "../../src/secp256k1v3-lib/der.ts" in source map of "../node_modules/ethereumjs-util/dist/secp256k1v3-lib/der.js".
⚠️  Could not load source file "../../src/secp256k1v3-lib/index.ts" in source map of "../node_modules/ethereumjs-util/dist/secp256k1v3-lib/index.js".
⚠️  Could not load source file "../../src.ts/coders/address.ts" in source map of "../node_modules/@ethersproject/abi/lib.esm/coders/address.js".
⚠️  Could not load source file "../../src.ts/coders/boolean.ts" in source map of "../node_modules/@ethersproject/abi/lib.esm/coders/boolean.js".
⚠️  Could not load source file "../../src.ts/coders/array.ts" in source map of "../node_modules/@ethersproject/abi/lib.esm/coders/array.js".
⚠️  Could not load source file "../../src.ts/coders/bytes.ts" in source map of "../node_modules/@ethersproject/abi/lib.esm/coders/bytes.js".
⚠️  Could not load source file "../../src.ts/coders/fixed-bytes.ts" in source map of "../node_modules/@ethersproject/abi/lib.esm/coders/fixed-bytes.js".
⚠️  Could not load source file "../../src.ts/coders/null.ts" in source map of "../node_modules/@ethersproject/abi/lib.esm/coders/null.js".
⚠️  Could not load source file "../../src.ts/coders/number.ts" in source map of "../node_modules/@ethersproject/abi/lib.esm/coders/number.js".
⚠️  Could not load source file "../../src.ts/coders/string.ts" in source map of "../node_modules/@ethersproject/abi/lib.esm/coders/string.js".
⚠️  Could not load source file "../../src.ts/coders/tuple.ts" in source map of "../node_modules/@ethersproject/abi/lib.esm/coders/tuple.js".
⚠️  Could not load source file "../src/fake.ts" in source map of "../node_modules/@aragon/wrapper/node_modules/ethereumjs-tx/dist/fake.js".
⚠️  Could not load source file "../src/transaction.ts" in source map of "../node_modules/@aragon/wrapper/node_modules/ethereumjs-tx/dist/transaction.js".
⚠️  Could not load source file "../src.ts/_version.ts" in source map of "../node_modules/@ethersproject/abi/lib.esm/_version.js".
⚠️  Could not load existing sourcemap of "../node_modules/@ethersproject/signing-key/lib.esm/elliptic.js".
⚠️  Could not load source file "../../src.ts/coders/abstract-coder.ts" in source map of "../node_modules/@ethersproject/abi/lib.esm/coders/abstract-coder.js".
⚠️  Could not load source file "../progress-event.ts" in source map of "../node_modules/xhr2-cookies/dist/progress-event.js".
⚠️  Could not load source file "../errors.ts" in source map of "../node_modules/xhr2-cookies/dist/errors.js".
⚠️  Could not load source file "../xml-http-request-upload.ts" in source map of "../node_modules/xhr2-cookies/dist/xml-http-request-upload.js".
⚠️  Could not load source file "../../src.ts/coders/anonymous.ts" in source map of "../node_modules/@ethersproject/abi/lib.esm/coders/anonymous.js".

I think it would be a good idea to open another issue regarding this