* 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.

Tests cannot run without built assets, and we want
exactly the matching assets. This is not a cache.

This replace calls of String#match? with rails Regex#match?
This follows the same idea used to keep Rails 5.2 compatible with Ruby
2.2.2 in https://github.com/rails/rails/pull/32973

* Disable simplecov on CircleCI

* Remove --format progress

* Port travis_retry for CI

* Add license