For Ubuntu edit plesk_atomic.py as IgorG said and then update as eilko said "sudo plesk php /usr/local/psa/admin/plib/DailyMaintainance/script.php -f UpdateModSecurityRuleSet daily"
For me it worked in ubuntu 14 + Onyx 17.0 + Atomic Basic & Ubuntu 16 + Onyx 17.5 + Atomic Basic