chore(grunt): fix [object Object] in banner

This is what the banner looks like today:

```javascript
/**
 * angular-strap
 * @version v2.0.0-rc.2 - 2014-01-29
 * @link http://mgcrea.github.io/angular-strap
 * @author [object Object]
 * @license MIT License, http://www.opensource.org/licenses/MIT
 */
```

This will change it to:

```javascript
/**
 * angular-strap
 * @version v2.0.0-rc.2 - 2014-01-29
 * @link http://mgcrea.github.io/angular-strap
 * @author Olivier Louvignes (olivier@mg-crea.com)
 * @license MIT License, http://www.opensource.org/licenses/MIT
 */
```