travis-ci: Include tests for multiple libgcrypt versions

[Lekensteyn]

The precise containers include libgcrypt 1.5 (libgcrypt11-dev), but some functionality require libgcrypt 1.6 (libgcrypt20-dev in trusty). For very new features, coverage for libgcrypt 1.7 would be nice as well.

If you would like to build libgcrypt from source:

• libgcrypt 1.4.2 depends on libgpg-error 1.4
• libgcrypt 1.5 depends on libgpg-error 1.9
• libgcrypt 1.6 depends on libgpg-error 1.11
• libgcrypt 1.7 depends on libgpg-error 1.13
• libgcrypt 1.8 (unreleased) depends on libgpg-error 1.25 (tentatively)

Edit: the updated travis CI files now use trusty which includes libgcrypt 1.5 and 1.6. The Windows builds (Appveyor) use libgcrypt 1.6.

[Lekensteyn]

With osx_image: xcode8.1, one could get Libgcrypt 1.7.3
https://blog.travis-ci.com/2016-11-15-xcode-81-is-here
https://docs.travis-ci.com/user/multi-os/

Edit: xcode8.2 is now used since 3bbc412

[tuxuser]

Hey,

I attempted a migration to GH Actions with limited success.

CI: https://github.com/tuxuser/luagcrypt/blob/ci/gh_actions/.github/workflows/ci.yml
Ref: https://github.com/tuxuser/luagcrypt/actions/runs/2567286750

NOTE: Windows build missing for now