JCBC-641: Upgrade Spymemcached to 2.11.5

Motivation
----------
Spymemcached 2.11.5 contains two important fixes, including one where
reconnect can take longer than necessary.

Modifications
-------------
Upgrade the dependency.

Change-Id: I40d4b73c9e0579088b5d35c284567d5168b06d9f
Reviewed-on: http://review.couchbase.org/43756
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>