Starting today, January 27, 2014, Canonical will no longer support the Ubuntu 13.04 (Raring Ringtail) operating system, and all users are urged to upgrade to Ubuntu 13.10 as soon as possible.
http://news.softpedia.com/news/Ubuntu-13-04-Is-No-Longer-Supported-Upgrade-to-Ubuntu-13-10-Now-420960.shtml