Migrate CI to GitHub Actions

[petergoldstein]

In addition to introducing a GitHub Actions configuration this PR:

1. Updates the minimum Ruby allowed for the gem to 2.5
2. Makes a similar change for the RequiredRubyVersion in the Rubocop config.

This minimum version change has been discussed with the maintainer.

Runs green on my fork.

[bquorning]

Two more things:

1. Shouldn’t we also remove .travis.yml in this PR?
2. There are a couple of RUBY_VERSION checks in ./Appraisals which are no longer necessary.

[mikegee]

i'm struggling to figure out how to make GitHub run these actions. does it need merged first?

[bquorning]

Yes, I think this first time, you need to merge it before it runs.

[petergoldstein]

@mikegee I believe so.

[mikegee]

Thanks!

[petergoldstein]

And the actions are running on master.