also delete /opt/couchbase-sync-gateway before building

Change-Id: Ie71e68912400bdcc2da2b63bbcb60d3ef73b263c
Reviewed-on: http://review.couchbase.org/40895
Reviewed-by: Philip Labee <philip@couchbase.com>
Tested-by: Philip Labee <philip@couchbase.com>