socket.io should listen to server, not app

[lchauvin]

I'm not sure how this works, but when I tried to reproduce this code, it didn't work.

I believe it's because socket.io is listening to Server(app), while it should listen to $server, which is what the server returns.

So I believe, the line 'var io = require('socket.io')(http)' should be moved after '$server = app.listen(3000, ...)', and should be modified to be 'var io = require('socket.io')(server)'