Support Ruby 2.7+ for v2.0

[AlexB52]

Maybe it's time to say goodbye to Ruby 2.5 and 2.6 now.

To be confirmed. This all started because MiniTest constant was undefined, but this was because I was running rake instead of bundle exec rake which means I was using a new version of Minitest that probably deprecates MiniTest as a possible constant since the gem is called "minitest" and not "mini_test"