Use Github Actions for CI

[RobinDaugherty]

With the restrictions that TravisCI has had to place on builds, we're unable to continue testing (because it's been taknig about 3 hours for Travis to run just one build).

This is a real shame, because Travis CI has been supporting free software projects like Better Errors for a long time. I hope they can find a way to get the abuse under control.

In the meantime, it looks best to switch this project to GitHub Actions, which has a more generous free tier.

• Coveralls reporting is no longer done using the coveralls gem, but using the simplecov-lcov formatter and the Coveralls GitHub Action to send reports and report the completion of the parallel build. Had to work around Fix incompatibility with simplecov < 0.18 fortissimo1997/simplecov-lcov#25 to get it to work with Ruby < 2.4 (which are only supported by older versions of simplecov). Followed this comment to set up Coveralls.
• GitHub Actions doesn't support "allow failure" builds, so I removed ruby-head, which is consistently failing today.
• Removed truffleruby from the matrix as well. It was always failing. I'll work to restore this and hopefully get it passing with binding_of_caller.