def migrate_down!(level = nil)
      migrations.sort.reverse.each do |migration|
        if level.nil?
          migration.perform_down()
        else
          migration.perform_down() if migration.position > level.to_i
        end
      end
    end
def migrate_down!(level = nil)
      migrations.sort.reverse.each do |migration|
        if level.nil?
          migration.perform_down()
        else
          migration.perform_down() if migration.position > level.to_i
        end
      end
    end
Run all the down steps for the migrations that have already been run.

 has an optional argument 'level' which, if supplied, only performs the
 down migrations with a postion greater than the level.