Ruby 3 in CI?

[digitalmoksha]

@minad what do you think about explicitly adding Ruby 3 into the Ci matrix? I know ruby-head is in there - not sure when the last pipeline was run, so I don't know if Ruby 3 was ever tested.

I tried it myself locally. Tests passed on both 3.0 and 3.1

I'm happy to put together a PR if you would consider it.

I'm curious as to whether travis is still working, or would we have to convert to GitHub actions.