MB-12185: Reference memcached from "couchbase" github remote

Change-Id: I1e3d40cb43fdc0afb446dc5a1576a2620a00a6d5
Reviewed-on: http://review.couchbase.org/41470
Reviewed-by: Chris Hillery <ceej@couchbase.com>
Tested-by: Chris Hillery <ceej@couchbase.com>