Upgrade to ISPConfig 3.2.8 fails when using wrong system php version 5.6

Hi,

Upgrade to ISPConfig 3.2.8 fails on php5.6:

# /usr/bin/php update.php --autoinstall=autoinstall.ini
PHP Fatal error:  Call to undefined function random_bytes() in /tmp/ispconfig_installer/ispconfig3_install/install/dist/conf/debian100.conf.php on line 68

I am not a PHP developer but as I see it, random_bytes() function is only available since PHP7, so the updater does not work with PHP5.6.

In ISPConfig you added a compatibility function in /usr/local/ispconfig/interface/lib/compatibility.inc.php, so I believe ISPConfig should still work on 5.6 after upgrade?

Thanks.