AbstractMethodError with Firebase Cloud Messaging

Update the version numbers of the Firebase libararies to be be consistent with the other Google Play Service libraries:

Change:

compile 'com.google.firebase:firebase-core:9.2.0'
compile "com.google.firebase:firebase-messaging:9.0.2"

To:

compile 'com.google.firebase:firebase-core:9.6.1'
compile 'com.google.firebase:firebase-messaging:9.6.1'