Use branch-20 for memcached

To include the following bugfix:
  * MB-10114: Don't remove items for allocation failures

Change-Id: Idc135fcfeab96dac7ce6d517ad2427b0d775ff08
Reviewed-on: http://review.couchbase.org/33977
Reviewed-by: Philip Labee <philip@couchbase.com>
Tested-by: Philip Labee <philip@couchbase.com>