升级Xcode9.3后 cocoapods 无法使用的解决办法

### Error

```

RuntimeError - [Xcodeproj] Unknown object version.

/Users/junyangyang/.rvm/gems/ruby-2.2.2@global/gems/xcodeproj-1.4.2/lib/xcodeproj/project.rb:217:in `initialize_from_file'

/Users/junyangyang/.rvm/gems/ruby-2.2.2@global/gems/xcodeproj-1.4.2/lib/xcodeproj/project.rb:102:in `open'

/Users/junyangyang/.rvm/gems/ruby-2.2.2@global/gems/cocoapods-1.2.0/lib/cocoapods/installer/analyzer.rb:851:in `block (2 levels) in inspect_targets_to_integrate'

/Users/junyangyang/.rvm/gems/ruby-2.2.2@global/gems/cocoapods-1.2.0/lib/cocoapods/installer/analyzer.rb:850:in `each'

/Users/junyangyang/.rvm/gems/ruby-2.2.2@global/gems/cocoapods-1.2.0/lib/cocoapods/installer/analyzer.rb:850:in `block in inspect_targets_to_integrate'

/Users/junyangyang/.rvm/gems/ruby-2.2.2@global/gems/cocoapods-1.2.0/lib/cocoapods/user_interface.rb:64:in `section'

/Users/junyangyang/.rvm/gems/ruby-2.2.2@global/gems/cocoapods-1.2.0/lib/cocoapods/installer/analyzer.rb:845:in `inspect_targets_to_integrate'

/Users/junyangyang/.rvm/gems/ruby-2.2.2@global/gems/cocoapods-1.2.0/lib/cocoapods/installer/analyzer.rb:66:in `analyze'

/Users/junyangyang/.rvm/gems/ruby-2.2.2@global/gems/cocoapods-1.2.0/lib/cocoapods/installer.rb:236:in `analyze'

/Users/junyangyang/.rvm/gems/ruby-2.2.2@global/gems/cocoapods-1.2.0/lib/cocoapods/installer.rb:150:in `block in resolve_dependencies'

/Users/junyangyang/.rvm/gems/ruby-2.2.2@global/gems/cocoapods-1.2.0/lib/cocoapods/user_interface.rb:64:in `section'

/Users/junyangyang/.rvm/gems/ruby-2.2.2@global/gems/cocoapods-1.2.0/lib/cocoapods/installer.rb:149:in `resolve_dependencies'

/Users/junyangyang/.rvm/gems/ruby-2.2.2@global/gems/cocoapods-1.2.0/lib/cocoapods/installer.rb:110:in `install!'

/Users/junyangyang/.rvm/gems/ruby-2.2.2@global/gems/cocoapods-1.2.0/lib/cocoapods/command/install.rb:37:in `run'

/Users/junyangyang/.rvm/gems/ruby-2.2.2@global/gems/claide-1.0.1/lib/claide/command.rb:334:in `run'

/Users/junyangyang/.rvm/gems/ruby-2.2.2@global/gems/cocoapods-1.2.0/lib/cocoapods/command.rb:52:in `run'

/Users/junyangyang/.rvm/gems/ruby-2.2.2@global/gems/cocoapods-1.2.0/bin/pod:55:in `<top (required)>'

/Users/junyangyang/.rvm/gems/ruby-2.2.2@global/bin/pod:23:in `load'

/Users/junyangyang/.rvm/gems/ruby-2.2.2@global/bin/pod:23:in `<main>'

```

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[!] Oh no, an error occurred.

Search for existing GitHub issues similar to yours:

https://github.com/CocoaPods/CocoaPods/search?q=%5BXcodeproj%5D+Unknown+object+version.&type=Issues

If none exists, create a ticket, with the template displayed above, on:

https://github.com/CocoaPods/CocoaPods/issues/new

Be sure to first read the contributing guide for details on how to properly submit a ticket:

https://github.com/CocoaPods/CocoaPods/blob/master/CONTRIBUTING.md

Don't forget to anonymize any private data!

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解决办法是升级升级cocoapods：gem install cocoapods --pre