Fixed Memcache driver configuration

This PR fixes several issues in Memcache driver configuration:

* Avoids to force default values in Memcached initialization, since all options available in Memcached PHP extension already have those.
* Avoids an exception happening with setting `OPT_RETRY_TIMEOUT` to 0 in Memcached 2.1.0
* Fixes servers configuration since [`Memcached` extension expect plain arrays as server spec](http://www.php.net/manual/en/memcached.addservers.php), not hashes (also see related https://github.com/tedivm/Stash/pull/71, which is *de facto* fixed by this PR).