Upgrade django-extensions for spatialite support

Older versions of django-extensions's reset_db command don't support
the spatialite database engine. This lets 'make demo' fail when using a
spatialite database.

This was fixed by our very own @skyjur in
https://github.com/django-extensions/django-extensions/commit/108b33919373e2f7a8f1fd41511eda6c1a7a3d9f