因后来ubuntu一直在升级，所以需要使用国内的镜像(关键)或旧版本源。

更新源

用你熟悉的编辑器打开：

/etc/apt/sources.list

1. 阿里云镜像 ★

deb https://mirrors.aliyun.com/ubuntu/ trusty main restricted universe multiverse
deb-src https://mirrors.aliyun.com/ubuntu/ trusty main restricted universe multiverse
deb https://mirrors.aliyun.com/ubuntu/ trusty-security main restricted universe multiverse
deb-src https://mirrors.aliyun.com/ubuntu/ trusty-security main restricted universe multiverse

deb https://mirrors.aliyun.com/ubuntu/ trusty-updates main restricted universe multiverse
deb-src https://mirrors.aliyun.com/ubuntu/ trusty-updates main restricted universe multiverse

deb https://mirrors.aliyun.com/ubuntu/ trusty-backports main restricted universe multiverse
deb-src https://mirrors.aliyun.com/ubuntu/ trusty-backports main restricted universe multiverse

2. Ubuntu旧版本的源

deb http://old-releases.ubuntu.com/ubuntu lucid main restricted universe multiverse    
deb http://old-releases.ubuntu.com/ubuntu lucid-security main restricted universe multiverse    
deb http://old-releases.ubuntu.com/ubuntu lucid-updates main restricted universe multiverse    
deb http://old-releases.ubuntu.com/ubuntu lucid-proposed main restricted universe multiverse    
deb http://old-releases.ubuntu.com/ubuntu lucid-backports main restricted universe multiverse    
deb-src http://old-releases.ubuntu.com/ubuntu lucid main restricted universe multiverse    
deb-src http://old-releases.ubuntu.com/ubuntu lucid-security main restricted universe multiverse    
deb-src http://old-releases.ubuntu.com/ubuntu lucid-updates main restricted universe multiverse    
deb-src http://old-releases.ubuntu.com/ubuntu lucid-proposed main restricted universe multiverse    
deb-src http://old-releases.ubuntu.com/ubuntu lucid-backports main restricted universe multiverse  

执行更新:

apt-get update

补充

1. 如果安装 gcc-4.8

sudo apt-get install gcc-4.8
sudo apt-get install g++-4.8

将 gcc-4.8 的优先级设置为 100

sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-4.8 100
sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-4.8 100

切换并查看当前 gcc 版本(该步非必须)

sudo update-alternatives --config gcc
sudo update-alternatives --config g++

参考：
https://developer.aliyun.com/mirror/ubuntu?spm=a2c6h.13651102.0.0.2f151b11Unnrjr
https://www.cnblogs.com/Segment-Tree/p/18764413