Temporary removed minitest from bundled gems.

  It requires Ruby 2.x.

  https://github.com/seattlerb/minitest/blob/master/Rakefile#L14

gems/bundled_gems

-minitest 5.14.1 https://github.com/seattlerb/minitest
 power_assert 1.2.0 https://github.com/ruby/power_assert
 rake 13.0.1 https://github.com/ruby/rake
 test-unit 3.3.6 https://github.com/test-unit/test-unit