Use BMemcached backend instead of PyLibMCCache

heroku/heroku-buildpack-python#201
heroku/heroku-buildpack-python#794

Looks like Heroku have a buggy version of libmemcached which meant that
pylibmc was failing to auth there with "FAILED TO SEND AUTHENTICATION TO
SERVER".

Switching to django-bmemcached as recommended by Memcached Cloud

https://devcenter.heroku.com/articles/memcachedcloud

emporium/emporium/settings.py

@@ -33,7 +33,7 @@
 if "MEMCACHEDCLOUD_SERVERS" in os.environ:
     CACHES = {
         "default": {
-            "BACKEND": "django.core.cache.backends.memcached.PyLibMCCache",
+            "BACKEND": "django_bmemcached.memcached.BMemcached",
             "LOCATION": os.environ.get("MEMCACHEDCLOUD_SERVERS", "").split(","),
             "OPTIONS": {
                 "username": os.environ.get("MEMCACHEDCLOUD_USERNAME"),