Switch to github actions

[ghiculescu]

This repo is experiencing pretty large CI backlogs at the moment. I'm guessing they are caused by issues @ Travis CI: https://news.ycombinator.com/item?id=25338983

This PR switches the CI over to Github actions, and removes Travis CI.

You can see outputs at https://github.com/ghiculescu/cancancan/actions (or at least I can). I think you'll need to enable something for the checks to show against this PR.

Other changes:

• I dropped Ruby 2.3 support. It's been EOL'd for over a year. https://www.ruby-lang.org/en/downloads/branches/
• I included a comment about this, but it's not clear to me why we have a failing test on jruby / AR 5.2. I haven't investigated yet. On Travis this combination was also allowed to fail, so I've excluded it in the test matrix here.
• I've included the changes in Remove unnecessary call to resolve_column_aliases #657 to run tests against Rails master. I've also added Rails 6.1. To get a green build here, I've included the single code change from Remove unnecessary call to resolve_column_aliases #657.
• I just can't get Nokogiri to work on Truffleruby, so I've excluded it until someone can figure that out.
• Same goes for Jruby + Rails 6.1. Here's a sample failure. Rails 6 is fine.

[ghiculescu]

@coorasse this is ready to go, what do you think?

[ghiculescu]

@coorasse FYI

[coorasse]

I am very happy of switching to GitHub Actions. I have a personal preference for SemaphoreCI but they lack support for build matrixes and exclude some combinations. 😢
Regarding Truffleruby I'd like to keep it running. @eregon can you help @ghiculescu with the nokogiri issue?

[eregon]

See oracle/truffleruby#62, so just adding this should fix it:

    - run: sudo apt-get -yqq install libxml2-dev libxslt-dev
      if: startsWith(matrix.ruby, 'truffleruby')

[ghiculescu]

Thanks for your help @eregon!

@coorasse we are now green on Truffleruby: https://github.com/ghiculescu/cancancan/actions/runs/431036514

[coorasse]

@ghiculescu could you please re-enable also Travis in this PR? I'd like to run both of them until I have a green build on Github Actions. Afterwards I'll remove the Travis configuration. Thank you! 🙏

[coorasse]

or maybe is enough to rebase and commit. I enabled Actions now.

[ghiculescu]

Hmmm. That doesn't seem to have worked :/

[ghiculescu]

@ghiculescu could you please re-enable also Travis in this PR?

Done @coorasse

[coorasse]

Nice! The first build has started 😄 https://github.com/CanCanCommunity/cancancan/actions/runs/452447924

[eregon]

Using bundler-cache: true would be much faster, I'll make a PR for that.

[eregon]

#682