[DJ1.9] Remove warning of not declaring explicit app_label

Example:

RemovedInDjango19Warning: Model class helios.models.Voter doesn't declare an explicit app_label and either isn't in an application in INSTALLED_APPS or else was imported before its application was loaded. This will no longer be supported in Django 1.9.

(cherry picked from commit 693acef9e4c3b093d5bf2bc1655e6f53a6dd4007)