* Add “bundle clean” suggestion from CircleCI doc

Cf. https://circleci.com/docs/2.0/caching/#bundler-ruby

* Use workspaces instead of caches for ruby gems

Cache are not guaranteed to be available, while the test jobs *require* their
depencies to run. Workspaces are thus more suitable.

One downside is that the order of workspace layer additions need to be
deterministic, which is why install-ruby{2.3,2.4} now depend on
install-ruby2.5.