change deprecated rack attack method

config/initializers/rack-attack.rb

@@ -74,7 +74,7 @@ class Rack::Attack
   # If you want to return 503 so that the attacker might be fooled into
   # believing that they've successfully broken your app (or you just want to
   # customize the response), then uncomment these lines.
-  self.throttled_response = lambda do |env|
+  self.throttled_responder = lambda do |env|
    [
      503,  # status
      {},   # headers