Get rid of hard-coded GCM sender ID

[rogerhu]

Require it instead as a config option is GCM is being used:

https://github.com/ParsePlatform/Parse-SDK-Android/blob/0c3009afd0070fc7867f94d3a11f38195382a27d/Parse/src/main/java/com/parse/GcmRegistrar.java#L42

[rogerhu]

Related to #572