Updated Gradle plugin version, Gradle version and Firebase libraries.

[jmslau]

This project is now compatible with Android Studio 2.2 Beta 1, which is currently the latest. Updating it for Android Studio 2.2 stable will be trivial.

[googlebot]

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[jmslau]

I am a Googler and I just added my @google.com email to my profile.

[googlebot]

CLAs look good, thanks!