Fix conversation list ordering

Order conversation list by timestamp instead of active_at. The former is
the send time of the most recent message. The latter is typically the
receive time of that message. This can cause mis-ordering if you send a
message while processing a backlog of incoming messages.

Fixes #617

// FREEBIE

js/conversation_controller.js

@@ -18,7 +18,7 @@
             this.listenTo(conversations, 'add change:active_at', this.addActive);
         },
         comparator: function(model) {
-            return -model.get('active_at');
+            return -model.get('timestamp');
         },
         addActive: function(model) {
             if (model.get('active_at')) {