Show exceptions during tests

By default rails sets `config.action_dispatch.show_exceptions = true` in test
environment, which suppresses exceptions in test log. This change reverses it so
exceptions are easier to see, thus making debugging easier.

Before:
![screen shot 2014-03-31 at 10 28 57 pm](https://cloud.githubusercontent.com/assets/116327/2575842/8ce4326e-b95e-11e3-8275-f04ed7f394ff.png)

After:
![screen shot 2014-03-31 at 10 29 48 pm](https://cloud.githubusercontent.com/assets/116327/2575844/a85b879a-b95e-11e3-82c6-842d8e7ce449.png)