Fixed breaking issue with non-file-based Session drivers

In Laravel, Session has a *SessionManager*. This class has a `__call()` method to send all it's function calls through its driver: *Store*. In the current code, `$app['store']` is being used in the service provider. When we switched the session driver to memcached, `$app['store']` wasn't loading all of the services it needed to use the session. When we switched this to `$app['session.store']` and switched the type hint in *Subscriber.php*, everything worked fine. This is because `$app['session.store']` uses the *SessionManager* instead of it's driver: *Store*.

We haven't tested this in Laravel 4.0 but it does resolve this issue in 4.1.