remove Rails dep by reading config path from ENV

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.
Merge pull request #2 from phinze/remove-rails-dependency

remove Rails dep by reading config path from ENV