CBLT-228:  add 4th param to build_sync_gateway (should be "enterprise" or "community")

use:  ./build_sync_gateway.sh   branch_name  version  platform  edition  [ OS ]  [ ARCH ]  [ DISTRO ]

LICENSE_${EDITION}.txt is copied to LICENSE.txt before calling packagers.

See sync_gateway commit:  eb3bbcbb295cd73d6f95739d623496116e10bdd0

Change-Id: I92b43c59609c2e523eaf7e2bb1dcf70a64a074bc
Reviewed-on: http://review.couchbase.org/35470
Reviewed-by: Philip Labee <philip@couchbase.com>
Tested-by: Philip Labee <philip@couchbase.com>