Find migrations files.

def get_migration_files(self):
        """Find migrations files."""
        migrations = (re.match(MigrationFile.PATTERN, filename)
                      for filename in os.listdir(self.directory))
        migrations = (MigrationFile(m.group('id'), m.group(0))
                      for m in migrations if m)
        return sorted(migrations, key=lambda m: m.id)