Wrong `relativeTo` for `FileSystemLoader`

[emilbayes]

It seems that the value / has been used as the relativeTo argument for the css FileSystemLoader causing issues with other postcss plugins such as postcss/postcss-import#167

The issue is here: index.js#L166. I don't know what the right value would be, but simply removing the argument resolves the referenced issue

[sandnuggah]

Any updates on this issue?