Use xmemtoy branch

Change-Id: I70e0e70f642e3c838cbb7e3dd207f01c98716730
Reviewed-on: http://review.couchbase.org/26532
Reviewed-by: Junyi Xie <junyi.couchbase@gmail.com>
Tested-by: Junyi Xie <junyi.couchbase@gmail.com>