fedora-cert: Fix checking against the CRL
    
    We're using two different APIs here:
    
    * OpenSSL.crypto.X509.get_serial_number(), which returns an int
    * OpenSSL.crypto.Revoked.get_serial() which returns a string containing
      the representation in hexadecimal of the serial
    
    Of course, when comparing them, we'd never find that any cert had been
    revoked.
    
    This patch makes sure we're not comparing apples to oranges.