def directories=(directories)
      valid = []

      if directories
        directories.each do |dir|
          unless File.directory?(dir)
            raise ArgumentError, "The directory #{dir} does not exist"
          end

          valid << File.expand_path(dir)
        end
      end

      @directories = valid
    end
def directories=(directories)
      valid = []

      if directories
        directories.each do |dir|
          unless File.directory?(dir)
            raise ArgumentError, "The directory #{dir} does not exist"
          end

          valid << File.expand_path(dir)
        end
      end

      @directories = valid
    end
Sets the directories to scan for external Ruby files using
 {RubyLint::FileLoader}.

 @param [Array] directories