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
Merge pull request #11 from virtru/claims-prn-to-claims-sub

changes claims.prn to claims.sub