Markdown: Upgrade redcarpet gem to 3.4 (#25781)

git-svn-id: http://svn.redmine.org/redmine/trunk@16583 e93f8b46-1217-0410-a6f0-8f06a7374b81

Gemfile

@@ -45,7 +45,7 @@ platforms :mri, :mingw, :x64_mingw do
 
   # Optional Markdown support, not for JRuby
   group :markdown do
-    gem "redcarpet", "~> 3.3.2"
+    gem "redcarpet", "~> 3.4.0"
   end
 end