CBLT-224:  use "macosx" instead of "Darwin", don't overwrite package with md5

Change-Id: I362c1e03b485b9a5c4bc054f91e69d8c4032c5ba
Reviewed-on: http://review.couchbase.org/33843
Reviewed-by: Philip Labee <philip@couchbase.com>
Tested-by: Philip Labee <philip@couchbase.com>