Add support for CAS

https://github.com/evan/memcached/pull/152 adds support for multi-CAS to the `memcached` gem. This adds support for both CAS and multi-CAS to the `memcached_store` gem.

This currently includes https://github.com/Shopify/memcached_store/pull/13 and hardcodes a dependency on the `multi_cas` branch of the `memcached` gem. Both will be removed before :ship:.

@camilo @csfrancis for review.