Add action for Ruby tests

[echeran]

Side note: I'm not sure why, but when using the Ruby action, changing directories in subsequent steps was not stateful in the way that that I remember seeing in other actions. Regardless, the combining the command-line steps into a single command works.

[sffc]

Could you add back the minimum supported Ruby version test that was in Travis?

[echeran]

Sure thing, it should be there now.

The new changes triggered a new workflow execution in my personal fork, but for some reason, it isn't executing here.

[sffc]

LGTM but please resolve the conflict; maybe that is what is preventing the CI from running?