rebaseTo setting is ignored

[plynchnlm]

I am trying to update grunt-contrib-cssmin to 3.0.0. Formerly, I could control the rebasing with the "root" option. I see clean-css 4 has replaced that with rebaseTo. However, when I change my Gruntfile to have:

      cssmin: {
        options: {
          rebase: true,
          rebaseTo: '<%= yeoman.client %>'
        }   
      },  

the URLs are rebased, but my rebaseTo setting is ignored, and resulting paths are relative to the destination file and point to the source image files (outside my dist directory).