added missing var declaration

app.js

@@ -7,7 +7,7 @@ var port = (process.env.PORT || 5000);
 var bpm = 120;
 var stepPos = 0;
 
-server.listen(port, function() {
+http.listen(port, function() {
     console.log('Listening on port %d', server_port);
 });