Remove incorrect file in rebase -i build step

Per https://github.com/github/training-kit/issues/73, this fixes the mystery file in the `rebase -i` where the "deleted" commit has an extra file listed.

![screen shot 2014-04-15 at 10 39 50 pm](https://cloud.githubusercontent.com/assets/352082/2712782/1d724732-c4de-11e3-8dba-1b46c4702390.png)
![screen shot 2014-04-15 at 10 39 44 pm](https://cloud.githubusercontent.com/assets/352082/2712783/1d750f80-c4de-11e3-8fa2-080b0d7f5f2c.png)