`config.rootDirectory` should be a String, not Array

I have got an exception:

```
 % grunt --stack crx
Running "crx:dev" (crx) task
Fatal error: path must be a string
TypeError: path must be a string
    at Object.fs.stat (fs.js:670:11)
    at /home/../grunt-crx/node_modules/crx/node_modules/wrench/lib/wrench.js:339:12
    at Object.oncomplete (fs.js:107:15)
```