CI: Organize matrix to run something

[olleolleolle]

This PR configures the build matrix to include newer versions of Rails.

• use an explcitly defined build matrix
• run Ruby 2.2-2.4 with Sprockets 3 on Rails 5.2, still works
• Remove rbx-3 - can not install

TODO

• [ ] Sprockets 4 support
• [ ] OR: Fail on Sprockets 4 support needed for the newer versions, move those to "allow failures"?

Other

Found a tiny issue, and extract its fix to https://github.com/rails/coffee-rails/pull/126.

[olleolleolle]

Or, is the better goal to work towards a Sprockets 4 configuration which works?