Stop sending emails about SuspiciousOperations

which are for invalid host headers.  See:

- http://www.tiwoc.de/blog/2013/03/django-prevent-email-notification-on-suspiciousoperation/
- https://code.djangoproject.com/ticket/19866
- http://stackoverflow.com/questions/15238506/djangos-suspiciousoperation-invalid-http-host-header