Fixed lost first option

when using a eslint.json file

```
{
  options: {
    config: "eslint.json"
  },
  target: "target.js"
}
```

[`eslint.execute`](https://github.com/sindresorhus/grunt-eslint/blob/master/tasks/eslint.js#L27) requires an argument as

`[ "node", "eslint", "--config", "eslint.json" ]`

and the first 2 elements are discarded.
(This behavior is [optionator](https://github.com/gkz/optionator) that are used in internal eslint.)

but got,

`[ "--config", "eslint.json" ]`

In this case, `options.config` is lost.

So, I added dummy elements.

`[ "", "", "--config", "eslint.json" ]`