Today marks the release of composer alpha4. This changes the default
stability from dev to stable, which means that if you want to use un-
stable dependencies, you either have to specify a root package level
minimum-stability, or use the @dev constraint, i.e. "1.0.*@dev".

We are setting the minimum-stability, so that we do not have to use
@dev for all dependencies, and all dependencies of dependencies.

We can increase the stability to beta once we release a beta of silex
(this will use Symfony 2.1 beta releases), and when we make a final
release (some time after Symfony 2.1.0), the flag can be removed.