Update conversation snippets automatically

Fixes stale snippets after the message has expired

js/views/conversation_list_item_view.js

@@ -20,6 +20,8 @@
             this.listenTo(this.model, 'change', _.debounce(this.render.bind(this), 1000));
             this.listenTo(this.model, 'destroy', this.remove); // auto update
             this.listenTo(this.model, 'opened', this.markSelected); // auto update
+            this.listenTo(this.model.messageCollection, 'add remove',
+              _.debounce(this.model.updateLastMessage.bind(this.model), 1000));
             extension.windows.onClosed(this.stopListening.bind(this));
             this.timeStampView = new Whisper.TimestampView({brief: true});
         },