Revert "Deactivesupport"

This reverts commit 4536cc2.

This has introduced a loading issue in kaminari-mongoid:
  kaminari/kaminari-mongoid#9

kaminari-core/lib/kaminari/models/configuration_methods.rb

@@ -1,10 +1,7 @@
 # frozen_string_literal: true
 module Kaminari
   module ConfigurationMethods #:nodoc:
-    def self.included(base)
-      base.extend ClassMethods
-    end
-
+    extend ActiveSupport::Concern
     module ClassMethods #:nodoc:
       # Overrides the default +per_page+ value per model
       #   class Article < ActiveRecord::Base

kaminari-core/test/fake_gem.rb

@@ -1,8 +1,10 @@
 # frozen_string_literal: true
 module Kaminari
   module FakeGem
-    def self.included(base)
-      def base.inherited(kls)
+    extend ActiveSupport::Concern
+
+    module ClassMethods
+      def inherited(kls)
         super
         def kls.fake_gem_defined_method; end
       end