Use a better cache

[alexgleason]

Info on caching in Django: https://docs.djangoproject.com/en/dev/topics/cache/

We're using the default cache, LocMemCache, which requires no special configuration but it's not as fast as other caches. We can speed this up by using Memcached or Redis.

I propose we alter settings/production.py to detect a REDIS_URL or similar environment variable and switch to using the proper service in the presence of that.