Fatal: Class ParsoidCacheUpdateTask not found

@Wikia/platform-team @inez 
https://wikia-inc.atlassian.net/browse/PLATFORM-354

Looks like there's a code path somewhere that's causing this task to be queued for a wiki that has $wgEnableVisualEditorUI = false. @inez - can someone on the contribution team investigate? For now, this will stop the fatals.