在 pod install 时提示错误：
### Error
```
RuntimeError - [Xcodeproj] Unknown object version.
/Library/Ruby/Gems/2.6.0/gems/xcodeproj-1.15.0/lib/xcodeproj/project.rb:227:in `initialize_from_file'
/Library/Ruby/Gems/2.6.0/gems/xcodeproj-1.15.0/lib/xcodeproj/project.rb:112:in `open'
/Library/Ruby/Gems/2.6.0/gems/cocoapods-1.9.1/lib/cocoapods/installer/analyzer.rb:1177:in `block (2 levels) in inspect_targets_to_integrate'
/Library/Ruby/Gems/2.6.0/gems/cocoapods-1.9.1/lib/cocoapods/installer/analyzer.rb:1176:in `each'
/Library/Ruby/Gems/2.6.0/gems/cocoapods-1.9.1/lib/cocoapods/installer/analyzer.rb:1176:in `block in inspect_targets_to_integrate'
/Library/Ruby/Gems/2.6.0/gems/cocoapods-1.9.1/lib/cocoapods/user_interface.rb:64:in `section'
/Library/Ruby/Gems/2.6.0/gems/cocoapods-1.9.1/lib/cocoapods/installer/analyzer.rb:1171:in `inspect_targets_to_integrate'
/Library/Ruby/Gems/2.6.0/gems/cocoapods-1.9.1/lib/cocoapods/installer/analyzer.rb:106:in `analyze'
/Library/Ruby/Gems/2.6.0/gems/cocoapods-1.9.1/lib/cocoapods/installer.rb:410:in `analyze'
/Library/Ruby/Gems/2.6.0/gems/cocoapods-1.9.1/lib/cocoapods/installer.rb:235:in `block in resolve_dependencies'
/Library/Ruby/Gems/2.6.0/gems/cocoapods-1.9.1/lib/cocoapods/user_interface.rb:64:in `section'
/Library/Ruby/Gems/2.6.0/gems/cocoapods-1.9.1/lib/cocoapods/installer.rb:234:in `resolve_dependencies'
/Library/Ruby/Gems/2.6.0/gems/cocoapods-1.9.1/lib/cocoapods/installer.rb:156:in `install!'
/Library/Ruby/Gems/2.6.0/gems/cocoapods-1.9.1/lib/cocoapods/command/install.rb:52:in `run'
/Library/Ruby/Gems/2.6.0/gems/claide-1.0.3/lib/claide/command.rb:334:in `run'
/Library/Ruby/Gems/2.6.0/gems/cocoapods-1.9.1/lib/cocoapods/command.rb:52:in `run'
/Library/Ruby/Gems/2.6.0/gems/cocoapods-1.9.1/bin/pod:55:in `<top (required)>'
/usr/local/bin/pod:23:in `load'
/usr/local/bin/pod:23:in `<main>'
```
原因：Xcode 版本和 CocoaPods 的版本不匹配，需要更新 CocoaPods
解决方法：
终端输入命令
gem install cocoapods --pre提示错误：
ERROR: While executing gem ... (Gem::FilePermissionError)
You don't have write permissions for the /Library/Ruby/Gems/2.6.0 directory.
加上权限：
sudo gem install cocoapods --pre再次执行pod install 成功！