Only load Railtie integration if Rails::Railtie is defined (#319)

Co-authored-by: Ufuk Kayserilioglu <ufuk.kayserilioglu@shopify.com>

Author: Morriar

lib/config.rb

@@ -79,7 +79,7 @@ def self.reload!
 end
 
 # Rails integration
-require('config/integrations/rails/railtie') if defined?(::Rails)
+require('config/integrations/rails/railtie') if defined?(::Rails::Railtie)
 
 # Sinatra integration
 require('config/integrations/sinatra') if defined?(::Sinatra)