Fix json decoding error on python3

response.content is returned as a byte array, and json.loads will choke on it, leading to a failed log-in attempt, which you'll see in the django logs as:

Failed to parse remote verifier response: `b'{"audience":"[redacted]","expires":1387148033218,"issuer":"gmail.login.persona.org","email":"[redacted]","status":"okay"}'`