In addition to adding scenarios for 5.2 and 6.0, I had to do some
cleanup to get everything working well:
* Replace string-based version checks with numeric ones, and update all
version checks to treat 6+ the same as 5 (except where necessary).
* Avoid touching `ActiveSupport::Deprecation::DeprecationProxy`
instances to fix a couple of warnings in Rails 6.
* Update Bundler as part of container setup, because Rails 6's config
seems to rely on a newer version of Bundler than the one that comes
with the Ruby image.
* Update the regexes in the `show-routes` spec to be compatible with 5.2
and up.
* Add a new code path to `show-routes` for Rails 6.
* Upgrade all bundles, just for hygiene.
