protected function downloadDrush8($destination) {
    $file = fopen($destination, 'w');
    $ch = curl_init();
    curl_setopt($ch, CURLOPT_URL,
      'https://github.com/drush-ops/drush/releases/download/8.1.15/drush.phar');
    curl_setopt($ch, CURLOPT_FAILONERROR, TRUE);
    curl_setopt($ch, CURLOPT_FOLLOWLOCATION, TRUE);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, TRUE);
    curl_setopt($ch, CURLOPT_FILE, $file);
    curl_exec($ch);
    curl_close($ch);
    fclose($file);
    $this->_chmod($destination, 0755);
  }