Explicitly depend on evenement/evenement

The code has an explicit dependency on evenement/evenement, hence we should explicitly define it as a dependency.

Currently, it's (implicitly) installed as part of a dependency of the React PHP components. IMHO this is an implementation detail that we should not rely on.

Updated dependency graph:
![graph-composer](https://cloud.githubusercontent.com/assets/776829/3268821/35f26c1a-f2e2-11e3-9dad-5cc9586138f6.png)