CBLT-283:  use short date for sync_gateway packages

Change-Id: I8f461487173c494be71f087bcd4090b0c07f973c
Reviewed-on: http://review.couchbase.org/36863
Reviewed-by: Philip Labee <philip@couchbase.com>
Tested-by: Philip Labee <philip@couchbase.com>