On stock Debian 9.9, "apt-get install php-xml" is needed to get anything to load. Here is the error you get without it: (Extension DOM is required. in /var/www/html/vendor/symfony/config/Util/XmlUtils.php:50)
Fixes #655. Downgraded symfony/filesystem to v3.4 (PHP 5.5.9 compatible). Added 7.0.8 as minimal PHP version for PHP7.
Added update message for PHP versions lower then PHP 7.