Broaden generic param of rejectedNotificationListener

This broadens the types of accepted RejectedNotificationListener in registerRejectedNotificationListener. This means that a single RejectedNotificationListener<ApnsPushNotification> could be used with e.g. PushManager<T1> and PushManager<T2> where T1, T2 extend ApnsPushNotification.

The cast static method could be put into a RejectedNotificationListeners class.

https://github.com/relayrides/pushy/issues/33