坑太多了，居然镜像服务器总是连不上，有时候可以，有时候不行，总是超时。
只能手动修改镜像地址了。
修改这个文件 /etc/apt/sources.list
修改成下面的内容。
deb http://mirrors.ocf.berkeley.edu/raspbian/raspbian/ jessie main contrib non-free rpi
#deb http://mirrordirector.raspbian.org/raspbian/ jessie main contrib non-free rpi
# Uncomment line below then 'apt-get update' to enable 'apt-get source'
#deb-src http://archive.raspbian.org/raspbian/ jessie main contrib non-free rpi
有找到很多其他的镜像地址，也可以试一试。居然还有阿里云的。
#deb http://mirrors.tuna.tsinghua.edu.cn/raspbian/raspbian/ wheezy main contrib non-free rpi
#deb-src http://mirrors.tuna.tsinghua.edu.cn/raspbian/raspbian/ wheezy main contrib non-free rpi
#deb http://mirrors.neusoft.edu.cn/raspbian/raspbian/ wheezy main contrib non-free rpi
#deb-src http://mirrors.neusoft.edu.cn/raspbian/raspbian/ wheezy main contrib non-free rpi
#deb http://mirrors.ustc.edu.cn/raspbian/raspbian/ wheezy main contrib non-free rpi
#deb-src http://mirrors.ustc.edu.cn/raspbian/raspbian/ wheezy main contrib non-free rpi
#deb http://mirror.devunt.kr/raspbian/raspbian/ wheezy main contrib non-free rpi
#deb ftp://ftp.kaist.ac.kr/raspbian/ wheezy main contrib non-free rpi
#deb http://mirrors.tuna.tsinghua.edu.cn/raspbian/raspbian/ wheezy main contrib non-free rpi
#deb http://mirror.nus.edu.sg/raspbian/raspbian/ wheezy main contrib non-free rpi
#deb http://mirrordirector.raspbian.org/raspbian/ wheezy main contrib non-free rpi
deb http://mirrors.aliyun.com/raspbian/raspbian/ wheezy main non-free contrib
deb-src http://mirrors.aliyun.com/raspbian/raspbian/ wheezy main non-free contrib
然后再更新
$ sudo apt-get update