Fixes #3988 - use require instead of autoload_once_paths

The `autoload_once_paths` were used incorrectly. From http://guides.rubyonrails.org/configuring.html:

> config.autoload_once_paths accepts an array of paths from which
> Rails will autoload constants that won't be wiped per request.
> Relevant if config.cache_classes is false, which is the case
> in development mode by default. Otherwise, all autoloading happens
> only once.
> All elements of this array must also be in autoload_paths.

In our case, the paths are not in autoload_paths directly.