Add Appraisal

Add appraisal to this way we can test agains different versions of `activesupport`, `activerecord`, `mongoid` etc.

I defined the default rake task so when run `$ rake` all the dependencies will be installed over the different gemfiles and run the tests in all the gemfiles.

It is possible to run a specifc gemfile with `$ appraisal rails-3.2 rake` for example.

The only gemfile we have all tests green is rails 4.1(`$ appraisal rails-4.1 rake`).

I propose that merge this PR, and the next steps focus on make the tests pass on all gemsets.

This way Travis will run this gemfiles too. And will be more easy to catch bugs.

What you guys think?