[-] MO : Followup : Don't execute crontasks if the module is disabled

To reproduce the bug:

1. Enable Follow Up module with at least one type of emails activated (eg: "re-order")
2. Set up a cron task on you server, executing Followup's cron.php
3. Disable the module
4. The cron is still executed but it doesn't check wether the module is activated or not