FIX: allowlisted_generic_onebox engine should be last

lib/onebox/engine.rb

@@ -9,7 +9,7 @@ def self.included(object)
     def self.engines
       constants.select do |constant|
         constant.to_s =~ /Onebox$/
-      end.map(&method(:const_get))
+      end.map(&method(:const_get)).sort_by { |klass| klass.to_s }.reverse
     end
 
     attr_reader :url, :uri