Replaced stability flags by branch alias

Replaced stability flags by branch alias as them only works for root package. For more info, refer to https://getcomposer.org/doc/04-schema.md#package-links.
By instance, I was trying to install dependencies for [SonataMediaBundle](https://github.com/sonata-project/SonataMediaBundle/blob/ee84fbe02f15e770db9435f7d56f18e0269593b2/composer.json#L25) as standalone package (for testing purposes), but I couldn't because the constraint used here.