def updated_at
      csv_time = google_key.nil? ? modification_time(uri) : google_modification_time
      (csv_time > yaml_time ? csv_time : yaml_time).to_s
    end
def updated_at
      csv_time = google_key.nil? ? modification_time(uri) : google_modification_time
      (csv_time > yaml_time ? csv_time : yaml_time).to_s
    end
The real +updated_at+ of a Table instance is the newer modification time of the csv file or
 the yaml file. Updates to either resource should break the cache.