[Rails5] Isolate `test_after_commit` gem

What does this MR do?

1. Adds unless condition for test_after_commit gem to the Gemfile. This gem has been integrated into rails 5.0. So it mustn't be included explicitly.
2. Update Gemfile.rails5.lock with the latest gem updates from the current rails4 GitLab repository by BUNDLE_GEMFILE=Gemfile.rails5 bundle update -j$(nproc) rugged loofah.

Are there points in the code the reviewer needs to double check?

No.

Why was this MR needed?

Migration to Rails 5.0

Screenshots (if relevant)

No.

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-and-qa manual pipeline job)

What are the relevant issue numbers?

#14286 (closed) and !12841 (closed)