Add a proper trigger config

[ci config]

.drone.yml

@@ -8,6 +8,9 @@ steps:
       remote: ssh://dokku@dokku.autonomic.zone:222/alerta
       ssh_key:
         from_secret: drone_deploy_key
-    trigger:
+trigger:
-      branch:
+  branch:
-        - master
+    - master
+  event:
+    exclude:
+      - pull_request