Not resolving to real file in sourceMap

When you enable the sourceMap option for node-sass, if you have:
```scss
@import "CSS:some_folder/some_file";
```
in some file,

you get
```json
...
"sources": [
  ...
  "...../CSS:some_folder/some_file",
  ...
],
...
```
in the sourceMap file.

I think `"...../CSS:some_folder/some_file"`  should be resolved to the path of the real file imported.
