Change our presence package

tmeasday:presence was doing unnecessary ping calls to the server every
5 seconds instead of using the status of the DDP connection (this
could save a fair amount of traffic in case of important server load).

I guess this change also fixes #221, but since no issue reproduction
was provided, it's difficult to tell.

server/publications/boards.js

@@ -141,7 +141,7 @@ Meteor.publishComposite('board', function(boardId) {
         // Presence indicators
         children: [{
           find(user) {
-            return Presences.find({userId: user._id});
+            return presences.find({userId: user._id});
           },
         }],
       },