更新为macOS 10.13之后cocoapods不能用了，网上搜索结果为就是重新安装一下
sudo gem install cocoapods
然后终端提醒我错误：ERROR: While executing gem ... (TypeError)
no implicit conversion of nil into String
于是更新gem
sudo gem update --system
更新成功后再次输入
sudo gem install cocoapods
这次错误变了 ：ERROR: While executing gem ... (Gem::FilePermissionError)
You don't have write permissions for the /usr/bin directory.
(说我没有写入/usr/bin的权限）
然后输入命令查看一下之前安装的pod的地址: which pod
发现地址为 /usr/local/bin
所以我们输入命令
sudo gem install -n /usr/local/bin cocoapods
发现更新成功了！
