Use the latest version of Bundler (Bundler 2) for RubyGem 3.0
Use the latest version of Bundler (Bundler 2) to avoid failures to install RubyGem 3.0+ on CI (test:install
/test:update
jobs).
https://bundler.io/blog/2019/01/03/announcing-bundler-2.html
Closes #435 (closed)