v1.7.0

• Feature: Update DNS Factory to accept complete Config object.
  Add new FallbackExecutor and use fallback DNS servers when Config lists multiple servers.
  (#179 and #180 by @clue)

  // old (still supported)
  $config = React\Dns\Config\Config::loadSystemConfigBlocking();
  $server = $config->nameservers ? reset($config->nameservers) : '8.8.8.8';
  $resolver = $factory->create($server, $loop);
  
  // new
  $config = React\Dns\Config\Config::loadSystemConfigBlocking();
  if (!$config->nameservers) {
      $config->nameservers[] = '8.8.8.8';
  }
  $resolver = $factory->create($config, $loop);