Skip padrino cache test if can not connect to cache server

- if can not connect to memcached, redis, mongo server,  which  skip those tests (padrino-cache/test/test_stores.rb)
- use memcached gem
 - because [memcache-client is already deprecated](https://github.com/mperham/memcache-client#memcache-client-is-deprecated-as-of-august-2010--it-will-be-supported-through-2010-but-new-code-should-use-dalli-instead) and already memcached gem in Gemfile