public function downloadDrupal7()
    {
        ProcessBuilder::create()
            ->setPrefix($this->drushBin)
            ->add('pm-download')
            ->add('drupal-7.38')
            ->add('--destination=' . $this->workingDir)
            ->add('--drupal-project-rename')
            ->add('--yes')
            ->setTimeout(600)
            ->getProcess()
            ->mustRun();
    }