Actually autoload all second-level directories called `app/*/concerns`

I'm proposing this change under the assumption that the [Constant Autoloading and Reloading](http://guides.rubyonrails.org/v4.2.0/constant_autoloading_and_reloading.html#autoload-paths) guide's note about the default `autoload_paths` should include:

> Any existing second level directories called `app/*/concerns` in the application and engines.

However, the actual `autoload_paths` only includes `app/controllers/concerns` and `app/models/concerns`.

This change actually does autoloading of all `app/*/concerns` directories.

I couldn't find any tests for `Rails::Engine::Configuration#paths`, so any guidance on test coverage for this change would be great!