Use elasticache-specific lib

The memcached lib does not support autodiscovery, so each memcached instance must be hard coded. Instead, use an elasticache-aware library, which should perform the auto-discovery for us.

@annalee @msecret - note that this python library has a c requirement. The next time you `pip install -r`, you'll want to first `apt-get install libmemcached-dev`. This is already part of the deployment scripts.