gulp-make-css-url-version when option {useDate: true} cause error

gulp-make-css-url-version when option {useDate: true} cause error

there's a bug in the index.js

the `date.now()` will cause the error, code should be changed to:

```
//use date as the version
            if (options.useDate) {
                return "url(" + url + "?v=" + formatDate(format, new Date()) + ")";
            }
```
