mem_cache_store requires dalli, so only accept dalli/client

`:mem_cache_store` require `Dalli`, rescue `Dalli` exceptions, and follow `Dalli` API.
Memcached gem, for instance, doesnt work anymore, as the API are different.

As we already require one client, we should make sure that client works, and not accept others, and if someone wants to use another memcache client they can write their own store adapter.

review @rafaelfranca @carlosantoniodasilva