原因：
- ubuntu15.04 已经不支持了，所以当初使用的镜像源都作废了，我们只需要更换软件源。
解决办法：
- sudo gedit /etc/apt/sources.list
- 粘贴以下几行，之后保存。然后sudo apt-get update 。大功告成。
-
p.s.同样的操作对其他ubuntu老版本也有效果，记得把vivid改成对应版本的版号。
deb http://old-releases.ubuntu.com/ubuntu vivid main restricted universe multiverse
deb http://old-releases.ubuntu.com/ubuntu vivid-security main restricted universe multiverse
deb http://old-releases.ubuntu.com/ubuntu vivid-updates main restricted universe multiverse
deb http://old-releases.ubuntu.com/ubuntu vivid-proposed main restricted universe multiverse
deb http://old-releases.ubuntu.com/ubuntu vivid-backports main restricted universe multiverse
deb-src http://old-releases.ubuntu.com/ubuntu vivid main restricted universe multiverse
deb-src http://old-releases.ubuntu.com/ubuntu vivid-security main restricted universe multiverse
deb-src http://old-releases.ubuntu.com/ubuntu vivid-updates main restricted universe multiverse
deb-src http://old-releases.ubuntu.com/ubuntu vivid-proposed main restricted universe multiverse
deb-src http://old-releases.ubuntu.com/ubuntu vivid-backports main restricted universe multiverse