move CI to github actions

[jrochkind]

This project is set up for CI on Travis, but Travis essentially doesn't provide free CI for open source anymore, and the CI doesn't currently appear to be running for this project.

I have had good experience with free Github Actions for CI. Reconstitute CI on Github Actions?

[mcritchlow]

Makes good sense to me. I've not yet used GH Actions, but might have time in a few weeks to take a pass at this, unless someone else wants to jump on it sooner.

[jrochkind]

Note CI won't currently pass in ruby 3.0, because of rexml dependency assumed in stdlib. #68