Update: Trigger content migrations on plugin update

[taylortom]

Update

• updatePlugin() now captures before/after plugin versions via readFrameworkPluginVersions
• Finds courses using the updated plugin and calls framework.migrateCourses() to run content migrations

Testing

• Depends on New: Run adapt-migrations directly for all trigger points adapt-authoring-adaptframework#174 for the migrateCourses() method and readFrameworkPluginVersions export
• Replaces the grunt-based approach from New: migration scripts to run on plugin update (fixes #28) #29

Fix

• n/a