Update Rouge to 3.18.0 (#32468).

git-svn-id: http://svn.redmine.org/redmine/trunk@19705 e93f8b46-1217-0410-a6f0-8f06a7374b81

Gemfile

@@ -5,7 +5,7 @@ gem 'bundler', '>= 1.12.0'
 
 gem 'rails', '5.2.4.2'
 gem 'sprockets', '~> 3.7.2' if RUBY_VERSION < '2.5'
-gem 'rouge', '~> 3.17.0'
+gem 'rouge', '~> 3.18.0'
 gem 'request_store', '~> 1.5.0'
 gem "mini_mime", "~> 1.0.1"
 gem "actionpack-xml_parser"