Travis config no longer valid

[gmarmstrong]

Travis CI has made some breaking changes in the last few years, the current build config is now invalid. Compare:

• April 2022 (fail): https://app.travis-ci.com/github/gmarmstrong/vim-muse/jobs/566538153/config
• September 2019 (pass): https://app.travis-ci.com/github/gmarmstrong/vim-muse/jobs/565711086/config

Since this is mostly just an issue with Travis (and since this project is not exactly mission-critical), I'm taking the Travis badge off the README for now, perhaps in violation of best practices.