The existing `project_name` implementation infers itself the directory the application exists in. However, this expectation fails several situations, such as

- Docker conventionally puts applications in /app
- Capistrano conventionally puts applications in /:prefix/project_name/releases/20181126-020459-399a6ff/

Instead, this infers the project name from the `Rails.application` class name, which is set by `rails new`. A similar approach is used in the marco-polo irb prompt gem:

https://github.com/arches/marco-polo/blob/0f53f5645bceac45757f061fc9b53ab2396afda7/lib/marco-polo.rb#L17

Fix for #105 (comment)

Prompt should not infer `project_name` from the path

Update the readme documentation

Use `module_parent_name` instead of `parent_name` for Rails 6.

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.