insert middleware before ActionDispatch::RequestId

see discussion in Octo-Labs#1

lib/heroku-request-id/railtie.rb

@@ -1,7 +1,7 @@
 module HerokuRequestId
   class Railtie < Rails::Railtie
     initializer 'heroku_request_id.add_middleware' do |app|
-      app.config.middleware.insert 0, "HerokuRequestId::Middleware"
+      app.config.middleware.insert_before "ActionDispatch::RequestId", "HerokuRequestId::Middleware"
     end
   end
 end