use branch-20 of libconflate in toy-junyi.xml

Change-Id: I391f80d8b9e47f90c9ba1b7f3d9e6d188f87b005
Reviewed-on: http://review.couchbase.org/24628
Tested-by: Junyi Xie <junyi.couchbase@gmail.com>
Reviewed-by: Junyi Xie <junyi.couchbase@gmail.com>