fix(config): don't alter process.env.CONFIG_FILES from the config script

fixes #1112.

@shane-tomlinson r?
@zaach - After looking at this, I think we can simplify the flow in 197-210 slightly:

```js
var DEV_CONFIG_PATH = ...
var files;

if (process.env.CONFIG_FILES && process.env.CONFIG_FILES.trim() !== '') {
  files = process.env.CONFIG_FILES.split(',');
} else if (fs.existsSync(DEV_CONFIG_PATH)) {
  files = [ DEV_CONFIG_PATH ];
}

if (files) {
  conf.loadFile(files);
}
```