Make travis only notify if the build changes status

Change the behaviour of travis to only notify if the build status
changes. The previous behaviour which was notifying for every build
regardless of status added a lot of unnecessary noise to the IRC
channel.

.travis.yml

@@ -13,5 +13,5 @@ notifications:
     irc:
         channels:
             - "chat.freenode.net#dunst"
-        on_success: always
+        on_success: change
         on_failure: always