Issues with running Joomla 3.8 and Civi
The fix to the file CRM/Utils/System/Joomla.php suggested in the link 
https://github.com/civicrm/civicrm-core/pull/11062/files?diff=unified
made the cron jobs run again for CiviCRM 4.7.25 and Joomla 3.8.1.