[CI] Run specs and tests

While we move away from rspec and towards minitest, now seems like a good time to start running both to make sure builds don't break.

cc @jch @schaary 

I'd like to drop official support for rbx for the 1.0 release as well, unless there's someone who's explicitly interested in supporting it. I don't think I can give it justice. Out of scope for this PR though.