force pickle session serializer (workaround for django 1.6+)

Django 1.6 switched to json as a serializer for session data, see https://docs.djangoproject.com/en/dev/releases/1.6/#default-session-serialization-switched-to-json . Unfortunately this breaks stachtach when a deployment other than 0 (all) is chosen in the web ui.

Explicitly setting pickle as a serializer works around this for now.