release only on drone promotion

.drone.yml

@@ -64,9 +64,11 @@ steps:
     commands:
       - (cd /tmp; go get -u github.com/go-bindata/go-bindata/...)
       - goreleaser release --rm-dist --parallelism=1
-    when:
+    trigger:
       event:
-        - tag
+        - promote
+      target:
+        - production
     depends_on:
       - submodules
       - clean