public static function configure() {
    $config = \BoletoSimples::$configuration;

    $oauth2 = new Oauth2Subscriber();
    $oauth2->setAccessToken($config->access_token);

    self::$client = new Client([
      'base_url' => $config->baseUri(),
      'defaults' => [
        'headers' => [
          'User-Agent' => $config->userAgent()
        ],
        'auth' => 'oauth2',
        'subscribers' => [$oauth2]
      ],
      'verify' => false
    ]);

    self::$default_options = ['headers' => ['Content-Type'=> 'application/json'], 'exceptions' => false];
  }