Revert "Disable Ruby 2.5 from CI due to bundler crashes

[MSP-Greg]

Description

This reverts commit a80a728af430b9b316e20221d69a21db4780af75.

'revisit when fixed'

Fixed by PR #1385

Completed Tasks

• [x] I have read the Contributing Guide.
• [x] The pull request is complete (implemented / written).
• [x] Git commits have been cleaned up (squash WIP / revert commits).
• [ ] I wrote tests and ran bundle exec rake locally (if code is attached to PR).