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 may break earlier installs of coffee-script.  The devDependencies are just for node-config testing, but lib/config.js is for everyone.  According that link, it looks like you can call a register function.  Would something like this work for both versions of coffee-script?
```
Coffee = Coffee || require('coffee-script');
if (Coffee.register) {
  Coffee.register();
}
```