Do not allow express 4, as dillinger is not compatible.

Otherwise you would get the following error when trying to launch the app:

```bash
app.configure(function(){
    ^
TypeError: Object function (req, res, next) {
    app.handle(req, res, next);
  } has no method 'configure'
    at Object.<anonymous> (/Users/crohr/dev/dillinger/app.js:15:5)
```