toy builders get PRODUCT_VERSION=%BUILD_NUMBER%-toy instead of -rel

Change-Id: I19aaa7f57b691c816298b5ab84bfb419389938e4
Reviewed-on: http://review.couchbase.org/39553
Reviewed-by: Philip Labee <philip@couchbase.com>
Tested-by: Philip Labee <philip@couchbase.com>