react-native-firebase

Update with Google Maven reference

I ran into this problem trying to follow your installation instructions.  The build kept failing with errors stating that it was unable to resolve the firebase packages. 

I could see that the 11.2.x packages weren't included in the latest Google Repository (58) and the latest release notes (https://developers.google.com/android/guides/releases) state that "Google Play services dependencies are now available via maven.google.com", so I added the Google Maven reference to the root build.gradle and it resolved the problem.