The extension is completely obsolote - please update the readme.

[andreibaca]

This extension uses multiplayer API and Drive API that are deprecated.
Multiplayer is nonexistant in new version of the library so it is not possible to build.
I tried to make it work by removing multiplayer: https://github.com/andreibaca/extension-googleplaygames
It builds but for other features the extension also uses deprecated methods that do not work.
Right now it is easier to create a new extension using the V2 of google play games services from scratch.

Please add some kind of deprecation info to the readme to spare the time of developers trying to use this.