Remove deprecated handler.register

wo/cli/plugins/maintenance.py

@@ -40,6 +40,6 @@ class WOMaintenanceController(CementBaseController):
 
 def load(app):
     # register the plugin class.. this only happens if the plugin is enabled
-    handler.register(WOMaintenanceController)
+    app.handler.register(WOMaintenanceController)
     # register a hook (function) to run after arguments are parsed.
     hook.register('post_argument_parsing', wo_maintenance_hook)