Update RuboCop Rails to 2.24 (#39889).

git-svn-id: https://svn.redmine.org/redmine/trunk@22772 e93f8b46-1217-0410-a6f0-8f06a7374b81

Gemfile

@@ -108,7 +108,7 @@ group :test do
   # RuboCop
   gem 'rubocop', '~> 1.60.0', require: false
   gem 'rubocop-performance', '~> 1.20.0', require: false
-  gem 'rubocop-rails', '~> 2.23.0', require: false
+  gem 'rubocop-rails', '~> 2.24.0', require: false
 end
 
 local_gemfile = File.join(File.dirname(__FILE__), "Gemfile.local")