Migrate to GitHub Actions

[petergoldstein]

This PR moves CI to GitHub Actions

In addition to setting up a basic configuration that includes those Rubies previously tested by Travis this PR:

1. Adds jruby-9.3 and Ruby 3.1 to the CI matrix
2. Loosens the Rake requirement in the gemspec to allow a Rake version that's compatible with ruby-head

[petergoldstein]

Looks like GitHub Actions needs to be enabled on this repo. On my fork this runs green. - https://github.com/petergoldstein/optimist/actions/runs/1842346534

[Fryguy]

Looks like GitHub Actions needs to be enabled on this repo. On my fork this runs green. - https://github.com/petergoldstein/optimist/actions/runs/1842346534

It's on by default in our org, but for some reason GitHub doesn't run it on the main repo until it's merged the first time. I presume to avoid people creating PRs with Bitcoin miners to unsuspecting repos. 🤷

[Fryguy]

Thank you so much!

[chessbyte]

Thank you @petergoldstein
@Fryguy do we need a follow-up PR to remove https://github.com/ManageIQ/optimist/blob/master/.travis.yml ?

[Fryguy]

Yes I added a note to the big checklist