Facebook iOS SDK 3.19

Summary:
    see https://developers.facebook.com/ios/change-log-3.x/

Upgrading:
    https://developers.facebook.com/docs/ios/upgrading/
Merge commit '307f6bcf986069987979fe66e30a424f8ecf4bb8'

* commit '307f6bcf986069987979fe66e30a424f8ecf4bb8':
  Facebook iOS SDK 3.19