Use RubyGems 3.x (3.0.2)

(Job Failed #140924004)

was fixed in !575 (merged).

---

Job #140924004 failed for 78ba3be1:

command -v bundler > /dev/null || gem install bundler
ERROR:  Error installing bundler:
	The last version of bundler (>= 0) to support your Ruby & RubyGems was 1.17.3. Try installing it with `gem install bundler -v 1.17.3`
	bundler requires RubyGems version >= 3.0.0. The current RubyGems version is 2.7.6. Try 'gem update --system' to update RubyGems itself.
Makefile:122: recipe for target 'bundler' failed
make: *** [bundler] Error 1

Refs.

• https://bundler.io/blog/2019/01/03/announcing-bundler-2.html

/cc @grzesiek