Add support for Mongoid4, drop support for Ruby 1.8.7.

I think it's time for Ruby 1.8.7 to go.

Got rid of appraisal so you don't need a separate Gemfile for each release and all this complicated infrastructure. Travis just runs it. You can just close this if you don't want it, but without appraisal you can do parallel builds on Travis, which also builds your PRs and I think it's better.