fix error

patchfile.gemspec

@@ -105,6 +105,7 @@ end
 #end
 begin
   require 'mongoid'
+  require "#{dir_pwd}/app/models/concerns/slug"
   require "#{dir_pwd}/app/models/site"
   site_first = Site.first
   if site_first[:display_title_in_frontend].nil?