fix(settings): fixes wrong cache config used for heroku

* Installs `django-heroku-memcacheify` only on heroku to 
  avoid installing `libmemcached-dev`[1] locally. 

[1] https://github.com/rdegges/django-heroku-memcacheify#install

Fixes: https://github.com/pydanny/cookiecutter-django/issues/80