CBLT-319:  fix package name for macosx sgw prepare-release stage

Change-Id: I93ed47d24371d96b2358f8e019b3aea3be2d7019
Reviewed-on: http://review.couchbase.org/38499
Reviewed-by: Philip Labee <philip@couchbase.com>
Tested-by: Philip Labee <philip@couchbase.com>