def bundle_dir=(new_dir)
      if new_dir
        new_dir = Pathname.new(new_dir)
        new_dir.mkpath
      end
      @bundle_dir = new_dir
    end
def bundle_dir=(new_dir)
      if new_dir
        new_dir = Pathname.new(new_dir)
        new_dir.mkpath
      end
      @bundle_dir = new_dir
    end
Set the `bundle_dir` where bundles will be written. If `bundle_dir` is set
 to nil, all bundles will be written to STDOUT