make junyi-toybuild use ep-engine toy branch

Change-Id: I67fe1f6d7a9e625ef5d1d66e5f9d53fea133f796
Reviewed-on: http://review.couchbase.org/25090
Reviewed-by: Junyi Xie <junyi.couchbase@gmail.com>
Tested-by: Junyi Xie <junyi.couchbase@gmail.com>