ci: migrate from Travis CI to GitHub Actions

[agilgur5]

Summary

Migrate CI from Travis to GitHub Actions, ensure tests run on macOS

• Fixes #54

Details

• Travis CI is only pseudo-free after the .org -> .com merge

• GH Actions are actually free for OSS / public repos

  • and also offer a decent level of composability that's resulted in a huge surge in reusable community Actions (vs. CircleCI orbs etc haven't had nearly as much adoption)
  • and I've had a mostly great experience on them so far as well

• add a matrix for OSes and different Go versions too

  • currently only using one Go version (as it was on Travis), but can add more later
  • currently only testing latest Ubuntu and macOS (as it was on Travis), but can add more versions and OSes later
    • note that tests fail on Windows right now (due to the shell?)

• check for $RUNNER_OS instead of $TRAVIS_OS_NAME now

  • update the commented out "Debug" step to conditionally run on macOS as well

fix: make get_docopts work on macOS

• tests were failing on macOS b/c sha256sum doesn't exist
  • only use the third arg in the workaround as it doesn't support flags

Testing

See CI success in my branch

Future Work

Will want to rename the travis directory to something like ci instead. I didn't do it in this PR as renames require a few changes and are easier to track in Git as a single commit -- I can do that in a separate PR after this is merged

[agilgur5]

Hmm, I'm guessing this has to be approved before CI runs on the PR? (per newer GitHub security features, and as this is the first workflow created and this is my first PR here, I think?)

[Sylvain303]

Seems good, lets merge that.