Update to GCM 8.1.0

[davidvavra]

GCM 8.1.0 introduced a breaking change and all libraries must update. Details:
https://www.reddit.com/r/androiddev/comments/3mehbb/breaking_changes_in_google_play_services_81/

[Epholl]

An updated version with GCM 8.1.0 on pull request...

[DanielNovak]

https://plus.google.com/u/0/+GoogleDevelopers/posts/HsSNWEQ6H4e

[DanielNovak]

Merged #57

[davidvavra]

@DanielNovak I think this affects only com.google.android.gms:play-services-analytics:8.1.0