remove Rails dep by reading config path from ENV

Yo @amireh - this is a cool project! :grinning:

I'm not sure if generating pretty static docs for a rubygem falls within the intended use case here - if not I'd love a pointer in the right direction to get slate-style docs into my gem. :+1: 

- - - 

Adds the ability to specify a path in the YARD_API_CONFIG environment
variable. This means yard-api no longer has to depend on Rails.root to
load itself up, meaning it can be used in a plain ole' Ruby gem.

Also removes use of `.present?` since that's an ActiveSupport-ism.

With these two tweaks, yard-api successfully generates docs from static
pages in my gem.