MAC OS 10.11之后更改了安全机制，所以cocoapods得安装和卸载命令也有所改变，
1、如果之前装过cocopods，最好先卸载掉，卸载命令：
$ sudo gem uninstall cocoapods【MAC OS 10.10之前】
$ sudo gem uninstall -n /usr/local/bin cocoapods【MAC OS 10.11之后】
2、先查看本地安装过的cocopods相关东西【本步骤没有必要】，命令如下：
$ gem list --local | grep cocoapods
会显示如下：
cocoapods-core (0.39.0)
cocoapods-downloader (0.9.3)
cocoapods-plugins (0.4.2)
cocoapods-search (0.1.0)
cocoapods-stats (0.6.2)
cocoapods-trunk (0.6.4)
cocoapods-try (0.5.1)
然后逐个删除吧：
$ sudo gem uninstall cocoapods-core【MAC OS 10.10之前】
$ sudo gem uninstall -n /usr/local/bin cocoapods-core【MAC OS 10.11之后】