Update the user's token if the GCM Result indicates a change of token

Per the GCM documentation, the token for a user can change at any point. If a change does occur, it is indicated in the Result returned from the server's send request. This pull request adjusts the user's GCM token if such a change is indicated.

The easiest way I found to test this is to have two different Sender IDs at your disposal.

1) Run the app as normal and register.
2) Modify the code of the Android app to register the original Sender ID plus your other. When you run the app, a new token is created by Google for both senders.
3) Send a notification from Jenkins. The original token still works, but the new token is provided as the one to use in the future.