setup_appraisals

Goal is to setup appraisals so we can properly test against multiple versions of rails, but also have travis test against mongo and datamapper, since we technically support those.

- [x] Get specs passing for 3.0 active record, datamapper, mongoid on Travis
- [x] ~~Add appraisals for 3.1, 3.2, 4.0~~ Moved to #14 

Some questions:

##### Does CanCan need `rails` for development?

Currently no, we only require active_record since we test mostly on PORO and only active_record in the adapter... We should add a dummy rails application and add some integration tests.

Needing rails does not mean needing `active_record`. We can have a rails app with datamapper and no active record

##### Rule asks for ActiveRecord::Relation

Currently datamapper and mongoid versions are bombing on two tests due to `lib/cancan/rule.rb:114`. This is adapter specific logic in the main app, but is too difficult to extract as it is. Best option might be to guard the condition with a `if defined? ActiveRecord` until the method can be refactored into small pieces and the entire condition be extracted to the adapter.