安装cocopod的时候会报错
/Library/Ruby/Gems/2.3.0/gems/cocoapods-1.6.1/lib/cocoapods/command.rb:118:in `git_version': Failed to extract git version from `git --version` ("xcrun: error: invalid active developer path (/Library/Developer/CommandLineTools), missing xcrun at: /Library/Developer/CommandLineTools/usr/bin/xcrun\n") (RuntimeError)
需要检查一下xcode是否安装命令行工具。
去Xcode - > Preferenced - > Locations并选择Command Line Tools,查看是否选择了当前xcode版本，就可以了
