Add missing import.

mayan/apps/converter/views.py

@@ -5,6 +5,7 @@ import logging
 from django.contrib.contenttypes.models import ContentType
 from django.core.exceptions import PermissionDenied
 from django.core.urlresolvers import reverse
+from django.http import Http404
 from django.shortcuts import get_object_or_404
 from django.utils.translation import ugettext_lazy as _