更新阿里yum源

1.下载阿里云的镜像源
wget -O /etc/yum.repo.d/Centos-Base.repo http://mirrors.aliyun.com/repo/Centos-7.repo
Centos6系统将Centos-7.repo改为Centos-6.repo

2.重新设置缓存
yum clean all
yum makecache