Migrate CI from Travis CI to GitHub Actions

[mishina2228]

Since June 15th, 2021, the building on travis-ci.org is ceased. This PR adds GitHub Actions workflows. Here are the details:

• Tests were being run on multiple patch versions(like 2.4.1, 2.4.5 and 2.4.6), now only the latest patch version will be tested.
• ruby/setup-ruby does not support Rubinius(rbx-2), so rbx-2 is not included in the CI matrix.
• Use Bundler v1.x for Rails 4 and earlier.
• The versions of Ruby and Rails in the test targets are outdated, but I migrated them as is.
• Comparison of CI jobs:
  • Travis CI https://travis-ci.org/github/nathanvda/cocoon
  • GitHub Actions https://github.com/mishina2228/cocoon/actions/runs/1765938505

[nathanvda]

Wow, great work. I will have to/want to look at this in a little more detail. Thanks a lot!

[mishina2228]

Once this PR is merged, I'll be happy to send PRs to clean up Travis and add new Ruby/Rails tests 😄