Class variable access from toplevel

[ericsaupe]

Fix warning found when starting up the app

https://github.com/solidusio-contrib/solidus_reviews/blob/master/app/helpers/spree/api/api_helpers_decorator.rb#L4

https://github.com/solidusio-contrib/solidus_reviews/blob/master/app/helpers/spree/api/api_helpers_decorator.rb#L10

warning: class variable access from toplevel

[aldesantis]

Closing, as I think we're good now.