fix rootpath without / at the end handling

less does not handle pathroot option without "/" at the end
e.g, ```data-rootpath=".."``` or ```-rp=..``` with this code
```css
backgroud-image: url("imgs/pic.png")
```
will outputs:
```css
backgroud-image: url("..imgs/pic.png")
```
instead of
```css
backgroud-image: url("../imgs/pic.png")
```
**NOTE:** I'm not sure if others options need this check too (e.g: --source-map-rootpath,...)