Fix cobertura filenames

Review after #11. Note that in your build #91, you see the same bug as in my issue #10.

![screen shot 2014-03-20 at 12 39 58 am](https://f.cloud.github.com/assets/526415/2469687/e02dac44-b002-11e3-9b20-55395ef1c5bd.png)

https://coveralls.io/files/153932109

But in this build, you should see the source code!

![screen shot 2014-03-20 at 1 09 03 am](https://f.cloud.github.com/assets/526415/2469832/ed1022f8-b006-11e3-8348-25d5ad8a01e7.png)

https://coveralls.io/files/157289978