Get GitHub actions running tests against Ruby 3.x

[carlwiedemann]

Right now there's a Travis CI file that doesn't appear to be active, and only mentions Ruby 2.x.

These days, most projects have moved to running GitHub actions for testing & linting.

If this project is indeed active (i.e. seeing recent activity from @sethk4783), It would be great to get that running for this project so it is clear that this project is compatible with Ruby 3.x.