[Rails5] Update ar5_batching initializer

What does this MR do?

Updates config/initializers/ar5_batching.rb to be compatible with rails 5.0

The exact changes are:

# New comments here
unless Gitlab.rails5?
  # Existing code here
end

Why was this MR needed?

Migrating to rails 5.0

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

#14286 (closed) and !12841 (closed)