Travis: Reduce IRC notification spam

Only send a "the build passed" notification if it previously didn't, i.e. the build was fixed.
Failure notifications stay the same

.travis.yml

@@ -9,3 +9,5 @@ notifications:
     channels: "chat.freenode.net#glowing-bear"
     use_notice: true
     skip_join: true
+    on_success: change
+    on_failure: always