Support all mongoid versions (2, 3, 4)

This pull request primarily adds back support for mongoid 2 and also fixes specs to pass with mongoid 4.   Travis test matrix was added in order to test mongoid 2, 3, and 4 against ruby 1.9.x, 2.0, jruby, and rubinius 2.  

All specs should be passing with the exception of mongoid 4 + rubinius 2 combo (which I'm allowing to fail in travis since mongoid 4 is still beta).