[Rails5] Update ar5_batching initializer

config/initializers/ar5_batching.rb

-# Port ActiveRecord::Relation#in_batches from ActiveRecord 5.
-# https://github.com/rails/rails/blob/ac027338e4a165273607dccee49a3d38bc836794/activerecord/lib/active_record/relation/batches.rb#L184
-# TODO: this can be removed once we're using AR5.
-raise "Vendored ActiveRecord 5 code! Delete #{__FILE__}!" if ActiveRecord::VERSION::MAJOR >= 5
-
-module ActiveRecord
+# Remove this file when upgraded to rails 5.0.
+unless Gitlab.rails5?
+  module ActiveRecord
     module Batches
       # Differences from upstream: enumerator support was removed, and custom
       # order/limit clauses are ignored without a warning.
@@ -39,4 +36,5 @@ module ActiveRecord
         end
       end
     end
+  end
 end