Fix necrobumping convos on key change

When inserting key change advisories, use the current conversation
timestamp to avoid pushing lots of old groups to the top of the
conversation list.

js/models/conversations.js

@@ -51,12 +51,11 @@
     },
 
     addKeyChange: function(id) {
-        var now = Date.now();
         var message = this.messageCollection.add({
             conversationId : this.id,
             type           : 'keychange',
-            sent_at        : now,
-            received_at    : now,
+            sent_at        : this.get('timestamp'),
+            received_at    : this.get('timestamp'),
             key_changed    : id
         });
         message.save();