`gem update --system` is not needed for for installing Bundler 2
`gem update --system` is not needed for for installing Bundler 2

* https://bundler.io/blog/2019/01/04/an-update-on-the-bundler-2-release.html
  requires RubyGems >= 2.5, and Ruby >= 2.3 ships with RubyGems 2.5m
  and Bundler does not support Ruby < 2.3.

dryrb/dry-monitor