On delivery receipts, reload messages only

The delivery receipt handler should only update messages, so rather than
reloading the conversation and its contacts, only reload the messages.

// FREEBIE

js/background.js

@@ -155,7 +155,7 @@
         function updateConversation(conversationId) {
             var conversation = ConversationController.get(conversationId);
             if (conversation) {
-                conversation.reload();
+                conversation.fetchMessages();
             }
         }