Add django-heroku-memcacheify to requirements and use it in the settings

I've added django-heroku-memcacheify to the requirements and now it's using it to get the env vars from the memcacheify in heroku.
It looks as though this can just be changed to `CACHES = memcacheify()`, as [`500` is the default](https://github.com/rdegges/django-heroku-memcacheify/blob/v0.8/memcacheify.py#L42), and it will [fall back to `LocMemCache`](https://github.com/rdegges/django-heroku-memcacheify/blob/v0.8/memcacheify.py#L86).