add braintree as a dependency since it's an undefined model with the gem

without requiring/loading the braintree gem, I got this.

![screen shot 2014-06-22 at 4 50 47 pm](https://cloud.githubusercontent.com/assets/492603/3352836/f43ada5c-fa5f-11e3-86a5-53b55eaadec5.png)