解决办法：
打开Ubuntu的终端,输入
sudo gedit /etc/apt/sources.list
再执行:
sudo apt-get update
就可以生效。
若是没有执行sudo apt-get update
结果老是按原来的源更新，重启了也没有用。
推荐一个不错的源（已经用以下源解决）：
deb http://mirrors.163.com/ubuntu/ oneiric main restricted universe multiverse
deb http://mirrors.163.com/ubuntu/ oneiric-security main restricted universe multiverse
deb http://mirrors.163.com/ubuntu/ oneiric-updates main restricted universe multiverse
deb http://mirrors.163.com/ubuntu/ oneiric-proposed main restricted universe multiverse
deb http://mirrors.163.com/ubuntu/ oneiric-backports main restricted universe multiverse
deb-src http://mirrors.163.com/ubuntu/ oneiric main restricted universe multiverse
deb-src http://mirrors.163.com/ubuntu/ oneiric-security main restricted universe multiverse
deb-src http://mirrors.163.com/ubuntu/ oneiric-updates main restricted universe multiverse
deb-src http://mirrors.163.com/ubuntu/ oneiric-proposed main restricted universe multiverse
deb-src http://mirrors.163.com/ubuntu/ oneiric-backports main restricted universe multiverse
deb http://archive.ubuntu.com/ubuntu/ oneiric main restricted universe multiverse
deb http://archive.ubuntu.com/ubuntu/ oneiric-security main restricted universe multiverse
deb http://archive.ubuntu.com/ubuntu/ oneiric-updates main restricted universe multiverse
deb http://archive.ubuntu.com/ubuntu/ oneiric-proposed main restricted universe multiverse
deb http://archive.ubuntu.com/ubuntu/ oneiric-backports main restricted universe multiverse
deb-src http://archive.ubuntu.com/ubuntu/ oneiric main restricted universe multiverse
deb-src http://archive.ubuntu.com/ubuntu/ oneiric-security main restricted universe multiverse
deb-src http://archive.ubuntu.com/ubuntu/ oneiric-updates main restricted universe multiverse
deb-src http://archive.ubuntu.com/ubuntu/ oneiric-proposed main restricted universe multiverse
deb-src http://archive.ubuntu.com/ubuntu/ oneiric-backports main restricted universe multiverse
