Use appraisal for gem version testing

[kmcphillips]

In #424 we talked about using appraisal for gem version testing.

https://github.com/thoughtbot/appraisal

[kmcphillips]

In discussions with Rafael we've decided:

I don’t see a need to add this gem
travis does that
I don’t think adding a gem to do a feature that already exist is the preferable
but I follow the philosophy: less code, better code