Xcode 9.3 pod install update 错误

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先查看pods版本：  pod --version

原因：这是因为你的 Xcode 版本和 CocoaPods 的版本不匹配，你需要更新你的 CocoaPods 
解决方法：终端输入命令 $ gem install cocoapods --pre

如果更新完还是不行  打开工程  这里的Project Format 默认为Xcode 9.3-compatible，将它修改9.3以下即可