Call Module#module_parent instead of deprecated #parent (#354)

Author: grk

lib/closure_tree/support.rb

@@ -32,7 +32,8 @@ def initialize(model_class, options)
     end
 
     def hierarchy_class_for_model
-      hierarchy_class = model_class.parent.const_set(short_hierarchy_class_name, Class.new(ActiveRecord::Base))
+      parent_class = ActiveSupport::VERSION::MAJOR >= 6 ? model_class.module_parent : model_class.parent
+      hierarchy_class = parent_class.const_set(short_hierarchy_class_name, Class.new(ActiveRecord::Base))
       use_attr_accessible = use_attr_accessible?
       include_forbidden_attributes_protection = include_forbidden_attributes_protection?
       model_class_name = model_class.to_s