Update RuboCop to 1.26 (#35136).

git-svn-id: http://svn.redmine.org/redmine/trunk@21452 e93f8b46-1217-0410-a6f0-8f06a7374b81

Gemfile

@@ -100,7 +100,7 @@ group :test do
   gem "selenium-webdriver", "~> 3.142.7"
   gem 'webdrivers', '4.6.1', require: false
   # RuboCop
-  gem 'rubocop', '~> 1.25.0'
+  gem 'rubocop', '~> 1.26.0'
   gem 'rubocop-performance', '~> 1.13.0'
   gem 'rubocop-rails', '~> 2.13.0'
 end