Do not notify Travis on_success

We don't find it useful anymore, because notifications from many other
CIs are mixed.

.travis.yml

@@ -543,7 +543,7 @@ notifications:
     rooms:
       - secure: ah7UEHBvncXT7bM5mvYIQAO+tIyV/wl7nXLb7wQD16dO2v8Gragy0mWjB79Q09hrrMGmp6H9bCDpdGS80boIA5EHaHoG4QaP0i9bsSt8U2AMWgZtfyIgQKJ4H2kXkGlrjO+AXTgnIkP7LNjdgAVUUTGQPb26T3QmoN2Splt+fIQ= # ruby:<token>#alerts
     on_pull_requests: false
-    on_success: change
+    on_success: never
     on_failure: always
 
   email: