Use a more efficient cache backend

[berkerpeksag]

According to https://github.com/python/psf-salt/blob/master/salt/pydotorg/config/django-settings.py.jinja, we use the default CACHE backend which is LocMemCache:

CACHES = {
    'default': {
        'BACKEND': 'django.core.cache.backends.locmem.LocMemCache',
    }
}

See https://docs.djangoproject.com/en/1.7/topics/cache/#local-memory-caching for details.

Switching to a more efficient cache backend might improve performance of django-sitetree as noted at http://django-sitetree.readthedocs.io/en/latest/performance.html. See #1048 for details.