Resolve dependencies conflicts with activerecord v 3.2+

`activerecord, ~> 3.1.0` is equal to `>=3.1.0 and < 3.2`, according to [documentation](http://docs.rubygems.org/read/chapter/16#page74) thus having activerecord v 3.2+ causes the issue. Hope it will be merged soon so we could switch back to original source.