Explicitly list dependencies

Currently, they're (implicitly) installed as part of a dependency of the other dependencies. IMHO this is an implementation detail that we should not rely on.

The resulting dependency graph looks like this:
![graph-composer](https://cloud.githubusercontent.com/assets/776829/3224338/b2df5aca-f031-11e3-8ff4-7cfeceb17aa0.png)