Ordering by created_at date is fraught with peril

Hah. Well, figured out why the random test failure was occuring.

Our tests are executed so quickly that we frequently create both of the
overheards at the exact same second!

I debugged this by:

1. Explicitely creating *two* overheards in the test for retrieving a
   list by json.
2. Asserting that the first overheard in the response is the second
   overheard created.
3. `p` ing out the json that was being outputted and visually inspecting
   the datetimes
4. [Facepalming](http://media.giphy.com/media/siFaJVXkUiJHO/giphy.gif)