修改ubuntu镜像源

修改配置文件（/etc/apt/sources.list）将安装源设置为国内的镜像，例如清华Ubuntu 16.04的镜像

deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial-updates main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial-backports main restricted universe multiverse
deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu/ xenial-security main restricted universe multiverse

可选的ubuntu国内源：
阿里云：http://mirrors.aliyun.com/ubuntu/
清华：https://mirrors.tuna.tsinghua.edu.cn/ubuntu/