CBLT-198:  each sync-gateway builder sets its SYNCGATE_VERSION_ param

Change-Id: Ie434586e2c280ecfd0746672c1fec8debc7307e8
Reviewed-on: http://review.couchbase.org/34128
Reviewed-by: Philip Labee <philip@couchbase.com>
Tested-by: Philip Labee <philip@couchbase.com>