pod 版本更新最新方法

淘宝的那个镜像(https://ruby.taobao.org/ )已经不可用了。所以我们现在用最新支持的ruby镜像(https://gems.ruby-china.org/)
需要的命令行：

$ gem sources -r https://rubygems.org/ （移除旧版本的镜像，如果你不知道你电脑上目前用的是什么镜像，可用  $ gem sources -l  来查看） 
$ gem sources -a https://gems.ruby-china.org/ （增加可用的镜像）  
$ gem sources -l  （用来检查使用替换镜像位置成功）
$sudo gem install  cocoapods (可能会报错)

ERROR:  SSL verification error at depth 2: unable to get local issuer certificate (20)
ERROR:  You must add /C=US/O=VeriSign, Inc./OU=Class 3 Public Primary Certification Authority to your local trusted store
ERROR:  SSL verification error at depth 1: unable to get local issuer certificate (20)
ERROR:  You must add /C=IL/O=StartCom Ltd./OU=Secure Digital Certificate Signing/CN=StartCom Certification Authority to your local trusted store
ERROR:  SSL verification error at depth 2: self signed certificate in certificate chain (19)
ERROR:  Root certificate is not trusted (/C=US/O=GeoTrust Inc./CN=GeoTrust Global CA)
ERROR: While executing gem ... (Errno::EPERM)
Operation not permitted - /usr/bin/pod

####执行以下命令
$ sudo gem install -n /usr/local/bin cocoapods
$ pod setup
$ pod --version

更新遇到问题的解决方案http://www.jianshu.com/p/9d15c8376b83