title: socket.io not working on heroku
body: <p>Whenever I try to use <code>require("socket.io");</code> on heroku it fails with the message "Cannot find module socket.io".</p>
  
  <p>I think it’s an issue with my setup, because the same is running fine in my local node.js server.</p>
  
  <p>What do I have to change?</p>