calendar: Fix thinko

Without an explicit index, messages should be added at the end of
the list.

https://bugzilla.gnome.org/show_bug.cgi?id=745988

js/ui/calendar.js

@@ -1289,7 +1289,7 @@ const MessageListSection = new Lang.Class({
     },
 
     addMessage: function(message, animate) {
-        this.addMessageAtIndex(message, 0, animate);
+        this.addMessageAtIndex(message, -1, animate);
     },
 
     addMessageAtIndex: function(message, index, animate) {