Fix for this.files undefined in grunt 0.4.0rc8

When using grunt 0.4.0rc8, <code>this.file</code> is undefined on line 17 of tasks/casperjs.js.

Have added a quick workaround to get things working. See [this comment](https://github.com/gruntjs/grunt/issues/606#issuecomment-11976240) for more info.