Cron 'Last-Run' value in admin area is always empty

## Bug Description
The cron **Last-Run** column in the admin area is always an empty value, despite cron jobs having been run.  

## How to Reproduce
Steps to reproduce the behavior:
1. Set up an automated cron job to be run
2. Visit e107_admin/cron.php
3. Last run is empty. 

## Expected Behavior
Display the date/time of the last time the cron job was run. 
