Fix blank lastMessage for non-text messages

For non text messages (ex: media messages and group updates), the
lastMessage field was being populated with empty string, resulting in an
empty message preview in the conversation list. Instead, display 'Media
message' or 'Updated the group', etc...

// FREEBIE

js/models/messages.js

@@ -307,12 +307,12 @@
                 if (!conversation_timestamp || message.get('sent_at') > conversation_timestamp) {
                     conversation.set({
                         timestamp: message.get('sent_at'),
-                        lastMessage: message.get('body')
+                        lastMessage: message.getDescription()
                     });
                 }
                 else if (!conversation.get('lastMessage')) {
                     conversation.set({
-                        lastMessage: message.get('body')
+                        lastMessage: message.getDescription()
                     });
                 }