Can't load library path files
Clearing the cache might help. I had tried to use libraries_flush_caches but that didn't seem to work. General CC did though - that will ensure libraries_load detects and loads the files you put in your files array. 

Also, for this library (at least composer version "google/apiclient": "1.0.*@beta") , you want to put 'autoload.php' in your files array - it'll do what it sounds like, auto-load all the classes you want to use instead of you manually having to do it.