Use Appraisal to make local testing with multiple Rails versions easier

This is basically the same PR as https://github.com/dazuma/rgeo-activerecord/pull/13 but for this project. I found using Appraisal makes it slightly easier to test against all the versions of Rails locally, so if this is useful, great, if not, feel free to ignore. And similar to the other PR, I also fixed rubinius testing and adding Ruby 2.1.