Using less's relativeUrls-option instead of manipulating the fileInfo directly

My solution introduced with ad8adde1c2acf69925f6f889cd4d38c62a7f173c is kind of a bit hacky, because the `fileInfo`-object is manipulated directly.

Now I found out that less is providing a `relativeUrls`-option which is rewriting all referenced urls and imports. It's basically the same as before, but the "official" way :wink: 

Additionally this fixes a small bug on windows where urls starting with `../` where not correctly resolved.