Fix relative paths when used in root .less file

When you give less an absolute rootpath, url paths end up being resolved to absolute filesystem paths when they appear in the root less file -- that is, `url(box.png)` becomes
`url(/path/to/less-loader/test/less/box.png)` when compiled. This was introduced in 0a1f6aee87d7c633fcd2635a23096d2a9e94f561; this should fix it.