Provisional Rails 4 updates; specs pass

 - Mongoid 4 uses upstream bson lib enforcing serialization with $oid;
   we unwrap by monkey-patching like promiscous ( http://git.io/cDladA )
 - Rspec 3 deprecations quieted
Just for me to understand correctly here.

Before to_json was returning a straight ID as a string.
Now mongoid 4 is returning `{'$oid' => 'abc'}`.
So the way to go around it is to proxy `to_json` to `to_s`?