update support for Coffee-script >=1.7.0

Starting from Coffee-script 1.7.0, the API for requiring `.coffee` in Node.js has changed as described in https://github.com/jashkenas/coffeescript/issues/3349#issuecomment-34015414 .

This patch updates the API for parsing config with the latest version of Coffee-script.

Tests run fine. 
This would call `register()` each time a file with `.coffee` extension is loaded. I don't know if that would cause any problems, but this seems better:
```
// Lazy load the coffee-script extension
if (!Coffee) {
  Coffee = require('coffee-script');
  if (Coffee.register) {
    Coffee.register();
  }
}
```