Fix function definition for on_upgrade_hook

It appears that #84 accidentally broke the post-upgrade hook, causing a critical error.
The upgrader_process_complete action accepts two arguments: the WP_Upgrader instance and an array of extra data. In the refactoring, the WP_Upgrader instance was removed, resulting in errors like:

Uncaught Error: Cannot use object of type SomePlugin as array in /path/to/wp-content/plugins/cache-enabler/inc/cache_enabler.class.php:420

• This topic was modified 4 years, 7 months ago by Luke Cavanagh. Reason: clarify wording
• This topic was modified 4 years, 7 months ago by Luke Cavanagh. Reason: clarify wording