Add Ruby 3.1 to CI

[petergoldstein]

As Ruby 3.1 is released, it should be added to CI as a formal entry.

[joshuacronemeyer]

thanks @petergoldstein. Merging this and merged a couple other PRs along with it to hopefully get back to a green build. Looks like the failures were from ruby >=3.1 removing some stuff from the standard lib that older nokogiris required. Updating nokogiri solved those failures.

[petergoldstein]

@joshuacronemeyer Great. I actually was planning on getting this running myself, but I'm glad you got it done.

[joshuacronemeyer]

I couldn't get our build passing for ruby 2.5. If you feel like it you could take a look at those build failures. Since ruby 2.5 support ended in 3/2021 I went ahead and removed it in favor of getting a passing build.

[petergoldstein]

I think removing Ruby 2.5 is the right move. It's EOL and Ruby 2.6 is EOLing in ~2 months.