Fix java.io.FileNotFoundException when @importing from LESS in subdirectory

[eager]

Imagine the following structure:

less
├── lib
│   ├── color.less
├── main.less
├── foo
│   ├── bar.less
└── foo.less

Contents of main.less:

@import "foo.less";

Contents of foo.less:

@import "foo/bar.less"

Contents of bar.less:

@import "../lib/color.less";

...

Without this patch, less-rhino-1.1.5.js fails to process bar.less, complaining File not found: path/to/less/../lib/color.less.