Fix compatibility with ActiveAdmin 1.4

[fixes #15]

Author: workgena

lib/active_admin_sidebar.rb

@@ -7,6 +7,7 @@ module ActiveAdminSidebar
   module Rails
     class Engine < ::Rails::Engine
       config.after_initialize do
+        ActiveAdmin::Views::Pages::Base.prepend ActiveAdminSidebar::ActiveAdminViewsPagesBase
         ActiveAdmin::BaseController.send :include, ActiveAdminSidebar::Positions
       end
     end

lib/active_admin_sidebar/activeadmin_views_pages_base.rb

@@ -1,4 +1,4 @@
-class ActiveAdmin::Views::Pages::Base < Arbre::HTML::Document
+module ActiveAdminSidebar::ActiveAdminViewsPagesBase
 
   def build_page_content
     build_flash_messages
@@ -9,6 +9,14 @@ def build_page_content
     end
   end
 
+  def build_sidebar
+    if defined?(super)
+      super
+    else
+      sidebar sidebar_sections_for_action, id: 'sidebar'
+    end
+  end
+
   def left_sidebar?
     assigns[:sidebar_options].try!(:[], :position) == :left
   end