Remove tests that are allowed to fail
Remove tests that are allowed to fail

This commit stops running tests against ruby-head. We mostly don't look
at these test results, and until there's some support for [allowed
failures][] I'd rather skip them so we can get a green build.

This commit also removes the failing jruby + Rails main combination,
although it leaves the CRuby + Rails main tests in place since they are
passing. Rails main is pretty stable, and folks are running it in
production, so I'd like to keep test coverage there if possible.

[allowed failures]: https://github.com/actions/toolkit/issues/399

thoughtbot/factory_girl