Update RuboCop Performance to 1.15 (#37247).

git-svn-id: https://svn.redmine.org/redmine/trunk@21803 e93f8b46-1217-0410-a6f0-8f06a7374b81

Gemfile

@@ -101,7 +101,7 @@ group :test do
   gem 'webdrivers', '4.6.1', require: false
   # RuboCop
   gem 'rubocop', '~> 1.36.0'
-  gem 'rubocop-performance', '~> 1.14.2'
+  gem 'rubocop-performance', '~> 1.15.0'
   gem 'rubocop-rails', '~> 2.15.0'
 end