switch to using memcached as the default caching backend for dashboard

The local memory cache causes bugs when running multiple apache
processes because each process has its own private cache[1]

https://docs.djangoproject.com/en/1.4/topics/cache/#local-memory-caching

Fixes bnc#829058 - dashboard project randomness
Merge pull request #92 from mapleoin/memcached

switch to memcached as the default caching backend