ISPConfig OS Update not working

short description

The OS Update function in ISPConfig stops working in Debian 9 stretch

correct behaviour

On Debian 9 Stretch the aptitude command does not work beacause aptitude isn't installed anymore in Debian 9.

environment

Server OS: (Debian 9.0)
Server OS version: (Stretch) ISPConfig version: (3.1.7)

proposed fix

use apt-get update && apt-get upgrade -y instead of aptitude