Switch GCM with FCM server endpoint
Switch GCM with FCM server endpoint

I purposedly left all comments, classes, debug output etc untouched,
because other than the server endpoint, users of the library still use
GCM and not FCM.

Renaming everything would break the API and thus require a version 3.

For reference, see:
https://developers.google.com/cloud-messaging/android/android-migrate-fcm#update_server_endpoints

aried3r/rpush,mandarin6b0/rpush,rpush/rpush,amaierhofer/rpush