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)