enable rss collection for memcached

Change-Id: Ieba0cb3030aa8775e386155283a9181eca08b54e
Reviewed-on: http://review.couchbase.org/42809
Reviewed-by: Venu Uppalapati <venumahesh@gmail.com>
Tested-by: Venu Uppalapati <venumahesh@gmail.com>