Add missing PermissionDenied import

apps/statistics/views.py

@@ -3,6 +3,7 @@ from __future__ import absolute_import
 from django.shortcuts import render_to_response
 from django.template import RequestContext
 from django.utils.translation import ugettext_lazy as _
+from django.core.exceptions import PermissionDenied
 
 from .api import statistics_functions