Rails5: fix Arel::UpdateManager in MigrateOldElasticsearchSettings migration

ee/changelogs/unreleased/rails5-fix-migration-update-manager.yml

+---
+title: 'Rails5: fix Arel::UpdateManager in MigrateOldElasticsearchSettings migration'
+merge_request: 6815
+author: Jasper Maes
+type: fixed

ee/db/migrate/20170215151540_migrate_old_elasticsearch_settings.rb

 class MigrateOldElasticsearchSettings < ActiveRecord::Migration
+  include Gitlab::Database::ArelMethods
   include Gitlab::Database::MigrationHelpers
 
   DOWNTIME = false
@@ -23,7 +24,7 @@ class MigrateOldElasticsearchSettings < ActiveRecord::Migration
       end
 
       updater =
-        Arel::UpdateManager.new(ActiveRecord::Base)
+        arel_update_manager
           .table(settings)
           .set(settings[:elasticsearch_url] => urls.join(','))
           .where(settings[:id].eq(id))