更改ubuntu 的源

deb http://mirrors.aliyun.com/ubuntu/ xenial main restricted
deb http://mirrors.aliyun.com/ubuntu/ xenial-updates main restricted
deb http://mirrors.aliyun.com/ubuntu/ xenial universe
deb http://mirrors.aliyun.com/ubuntu/ xenial-updates universe
deb http://mirrors.aliyun.com/ubuntu/ xenial multiverse
deb http://mirrors.aliyun.com/ubuntu/ xenial-updates multiverse
deb http://mirrors.aliyun.com/ubuntu/ xenial-backports main restricted universe multiverse
deb http://mirrors.aliyun.com/ubuntu/ xenial-security main restricted
deb http://mirrors.aliyun.com/ubuntu/ xenial-security universe
deb http://mirrors.aliyun.com/ubuntu/ xenial-security multiverse