Compatibility with Rails 6 and Zeitwerk loader
Compatibility with Rails 6 and Zeitwerk loader

This change seems to fix the problem in Rails 6 `production` env, when zeitwerk tries to eager-load all the code and can't find the constants RailsWarden::Version. I didn't have any problems in `development` env, for some reason, though.

```
[...]
/home/florian/.rvm/gems/ruby-2.6.5@default/gems/zeitwerk-2.4.0/lib/zeitwerk/loader/callbacks.rb:17:in `on_file_autoloaded': expected file /home/florian/.rvm/
gems/ruby-2.6.5@default/bundler/gems/rails_warden-fb45d162be44/lib/rails_warden/version.rb to define constant RailsWarden::Version, but didn't (Zeitwerk::Nam
eError)
hassox/rails_warden