The master branch is required Ruby 2.2.2+, for the Ruby 2.2 is bundled Minitest 5.4.3,
I think backward compatibility code for Minitest 4 is unnecessary.
Tests on Rails [are currently failing](https://travis-ci.org/rails/rails/jobs/78255666).
The reason is the dependency of Rails master from gems that are currently on
GitHub (not on RubyGems) and should be explicitly referenced in the Guides
test files.