Sort migrations if necessary and store in self._sorted_migrations

def sorted_migrations(self):
        """
        Sort migrations if necessary and store in self._sorted_migrations
        """
        if not self._sorted_migrations:
            self._sorted_migrations = sorted(
                self.migration_registry.items(),
                # sort on the key... the migration number
                key=lambda migration_tuple: migration_tuple[0])

        return self._sorted_migrations