Making get_blacklisted_response work for both GET and POST.

Previously, this helper only worked if Twilio POSTed to us. In preparation for
supporting both GET and POST requests from Twilio, I'm updating this helper to
analyze HttpResponse.REQUEST as opposed to HttpResponse.POST, this way it'll
work in both cases.

django_twilio/utils.py

@@ -18,7 +18,7 @@ def get_blacklisted_response(request):
         otherwise.
     """
     try:
-        caller = Caller.objects.get(phone_number=request.POST['From'])
+        caller = Caller.objects.get(phone_number=request.REQUEST['From'])
         if caller.blacklisted:
             r = Response()
             r.reject()