Fix call to instance method from class method

Author: xuanxu

app/lib/responder_registry.rb

@@ -93,7 +93,7 @@ def self.available_responders
         responder_class = Object.const_get(name)
         available_responders[responder_class.key] = responder_class
       rescue NameError => err
-        logger.warn("There is a mismatch in a Responder class name/module: #{err.message}")
+        Logger.new(STDOUT).warn("There is a mismatch in a Responder class name/module: #{err.message}")
       end
     end