Update create_tarball.sh for 3.0.x (and future?) releases.

Change-Id: Icf9b92a356f2645a27be52c6aec63a4bc4d9d862
Reviewed-on: http://review.couchbase.org/44328
Reviewed-by: Chris Hillery <ceej@couchbase.com>
Tested-by: Chris Hillery <ceej@couchbase.com>