define feature files via `src` to get file with this.fileSrc

Hi

I was wondering about why grunt tries to find my `.feature` files at the default path though I defined one. After looking into the grunt-cucumber-js code, I saw that you are using `this.fileSrc` to get the file definition from the files attribute. That is not correct, see [here](http://gruntjs.com/api/inside-tasks).

> this.filesSrc
>
> In a multi task, all `src` files files specified via any file format are reduced to
> a single array. If your task is "read only" and doesn't care about destination
> filepaths, use this array instead of `this.files`.

Maybe this wasn't changed after migrating the plugin from grunt v0.3 to v0.4. Hope you'll find time to merge this PR. Anyway, thanks for your work!