Add a simple spec for validating presence of an _order field.

This is the same issue as #51, but I've added a breaking test.

Basically, `before_save :handle_ranking` gets run only **after** the validation throws the error. This is because validation callbacks happen first:

![screen shot 2014-03-02 at 11 59 47 pm](https://f.cloud.github.com/assets/590450/2306684/ab21aa0c-a290-11e3-8eff-4635559130b1.png)