Migrate CI to GitHub Actions

[petergoldstein]

This PR migrates CI from TravisCI.org, which is no longer operational, to GitHub Actions. In addition to basic setup it:

1. Adds a number of more recent Ruby and Rails versions
2. Loosens the restrictions in the gemspec to enable those more recent versions to bundle
3. Makes minor changes to the dummy Rails app to allow it to work with Rails 7
4. Conditionally explicitly includes the net-* mail gems for the Ruby 3.1 / Rails 6.1.x combination
5. Switches the Rails references from master to main in line with the rails/rails renaming of that branch.

Everything runs green.

I didn't attempt to pull over the Code Climate integration, although it looks like there's a suitably packaged action here that would be easy enough to incorporate.