avoid dependency conflict between play-services and firebase

[haraldreingruber]

As this seems to be a recurring issue, I would like to trigger a discussion about how this problem can best be mitigated for the future.

There are already several PRs offering different solutions (#323, #332 and #290), and according to #134 some users have other depending libraries which include a version (or the latest version?) of play-services on their own.

To my opinion, the most important thing is that the versions of these dependencies are consistent. So, I would be in favor of either setting all to a fixed version (#332), or all to the latest version (#323). Of course, if we set all to latest, life might be easier for users who have other dependencies to play-services...

What do you think?

[Salakar]

Wrote some docs on that here as it's generally a common issue

[haraldreingruber]

@Salakar: thanks for the hint.