CentOS修改镜像源头

CentOS6改为阿里镜像源：

1. cd /etc/yum.repos.d

2. mv CentOS-Base.repo CenOS-Base.repo.bak

3. wget http://mirrors.aliyun.com/repo/Centos-6.repo

4. mv Centos-6.repo CentOS-Base.repo

5. yum makecache