[CI] Test against Ruby 2.5

[nicolasleger]

Summary

All in title

Details

Motivation and Context

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Refactoring (cleanup of codebase without changing any existing functionality)
• Update documentation

Checklist:

• I've added tests for my code
• My change requires a change to the documentation.
• I have updated the documentation accordingly.