Update download dependency

Use the new version of download. One of the tests is going to break because of this [#L47-L79](https://github.com/yeoman/generator/blob/master/test/fetch.js#L47-L79). Not sure what the logic behind that test is (?).

Also, this doesn't take an array of urls anymore. We can add support for that by doing something like:

```js
url.forEach(function (item) {
    download.get(item, destination);
}
```
Also, it should be without _should_ -> `it('fetch a single file')`