Dependency issues on Android

Hi.

I have found some trouble when import the library in my current workspace. You are linking to the latest firebase-core/firebase-analytics version using the "+" operator in the Gradle file (compile 'com.google.firebase:firebase-core:+', compile 'com.google.firebase:firebase-analytics:+'). Two things about this: firebase-analytics is inside the core package(see documentation: https://firebase.google.com/docs/android/setup), the other is that the "+" operator is discouraged (android studio warns about it), instead you must use a concrete versión of the library.
I think that you must change this operator and provide different versions for every new release of firebase library, thus the project will have dependencies over a concrete version, not the latest, who can break the workspace code.
If you cannot made it, i can help you with some pull requests.

Best regards.