Fixed Source Map generation when multiple input/output paths are used.

[mkempe]

Hey there,

this is the PR for fixing the problem with multiple input output paths.

This probably fixes #32 and makes

lessOptions: {
  mergeTrees: {
    overwrite: true
  }
}

no longer necessary.

The problem is gabrielgrant/broccoli-less-single#29 and we have to wait until the problem is fixed and merged. This PR updates the README and the broccoli-less-single dependency for testing purpose.

[knownasilya]

Looks like that does the trick.

[knownasilya]

Once you update the version and squash, I can merge this. 👍

[mkempe]

Hey Ilya,

done, thanks again for looking into it.