Fix separator issue on Windows

The following test fails on Windows because `path.relative()` returns a string with `"\\"` separator.

```
Testing relative_root.test.jsF.
>> relativeRoot - simple
>> Message: relativizes absolute paths in CSS
>> Error: 'body {\n  background-image: url(\'..\\../images/bg.png\');\n}' == 'body {\n  background-image: url(\'../../images/bg.png\');\n}'
>> at Object.exports.relativeRoot.simple (test\relative_root.test.js:34:10)
>> at Object.exports.relativeRoot.setUp (test\relative_root.test.js:28:5)
```

I fixed the separator issue using [slash](https://www.npmjs.org/package/slash).
Thanks for providing this useful plugin!