[Rails5] Add rails5 jobs to gitlab-ci.yml

What does this MR do?

Adds rails5 jobs for both rspec and spinach tests to gitlab-ci.yml.

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

Check if this gitlab-ci.yml approach fits as best as possible.

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)