changes claims.prn to claims.sub

according to the Google documentation as of 04/24/2014:

Additional Claims:
sub - The email address of the user for which the application is requesting delegated access.

https://developers.google.com/accounts/docs/OAuth2ServiceAccount#additionalclaims


I manually tested this locally and both `claims.prn` and `claims.sub` worked successfully. Perhaps Google deprecated `claims.prn` and just has not removed it yet? 

I do not have an automated way to test this.