use rails root join

lib/gitlab/import_export.rb

@@ -21,7 +21,7 @@ module Gitlab
     end
 
     def config_file
-      File.join(Rails.root, 'lib/gitlab/import_export/import_export.yml')
+      Rails.root.join('lib/gitlab/import_export/import_export.yml')
     end
 
     def version_filename