[Rails5] Fix Gitaly client timeout

What does this MR do?

When BUNDLE_GEMFILE environment variable is explicitly set to non-default Gemfile it leaks to gitaly-ruby which breaks gitaly client to response. So it falls into timeout.

This MR removes the BUNDLE_GEMFILE variable in the Bundler.with_original_envblock so gitaly-ruby does not receive BUNDLE_GEMFILE anymore.

Tests for rails5 branch now should be green again.

Also Gemfile.rails5.lock is updated.

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

Gitaly client should work as good as it did before this change.

Why was this MR needed?

Fixes broken CI pipelines for rails5 branches.

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)

Closes gitaly#1169 (closed)