Listeners for events from other plugins break on plugin upgrade or UPM upgrade

ScriptRunner listeners for events from 3rd party plugins stop firing when either the UPM is upgraded, or the related plugin is upgraded. 

Steps to reproduce:

1. Bring up a Jira 8.0.0 instance with scriptrunner installed
2. Configure a listener for all projects and the SprintStartedEvent
3. Trigger it by starting a sprint
4. The listener will trigger.
5. Go to the manage apps page and click the link at the top asking to upgrade the “Atlassian Universal Plugin Manager Plugin” to 4.0.17
6. Try to trigger the listener again by starting a sprint, and it no longer triggers the listener.

Workaround:

• Restart Jira, reinstalling ScriptRunner will not fix the problem.