use memcached gem

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
if can not connect memcached, skip memcache with memcached server tests
if can not connect redis, skip redis with redis server tests
divide skip cause for Mongo test

- cause 1 : Mongo lib LoadError
- cause 2 : can not connect to Mongo server
Merge branch 'bash0C7-skip-padrino-cache-test-if-can-not-connect-to-cache-server'

Conflicts:
	padrino-cache/test/test_stores.rb