升级xcode9.3后创建工程pod init失败

如题，使用xcode9.3后新建工程，然后初始化pod init，结果报错：

报错信息

pod init

――― MARKDOWN TEMPLATE ――――――

### Command

```

/usr/local/bin/pod init

```

### Report

* What did you do?

* What did you expect to happen?

* What happened instead?

### Stack

```

   CocoaPods : 1.3.1

        Ruby : ruby 2.3.3p222 (2016-11-21 revision 56859) [universal.x86_64-darwin17]

    RubyGems : 2.6.14

        Host : Mac OS X 10.13.4 (17E202)

       Xcode : 9.3 (9E145)

         Git : git version 2.15.1 (Apple Git-101)

Ruby lib dir : /System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/lib

Repositories : master - https://github.com/CocoaPods/Specs.git @ 8e9a8f04aa2579345661be7afc6f6fafdfb09d51

               PrivatePods - https://github.com/ModulizationDemo/PrivatePods.git @ 22ad9a433c6e02afeef4d32da95ba0fe64799a0d

```

### Plugins

```

cocoapods-deintegrate : 1.0.1

cocoapods-plugins    : 1.0.0

cocoapods-search      : 1.0.0

cocoapods-stats      : 1.0.0

cocoapods-trunk      : 1.3.0

cocoapods-try        : 1.1.0

```

### Error

```

RuntimeError - [Xcodeproj] Unknown object version.

/Library/Ruby/Gems/2.3.0/gems/xcodeproj-1.5.3/lib/xcodeproj/project.rb:217:in `initialize_from_file'

/Library/Ruby/Gems/2.3.0/gems/xcodeproj-1.5.3/lib/xcodeproj/project.rb:102:in `open'

/Library/Ruby/Gems/2.3.0/gems/cocoapods-1.3.1/lib/cocoapods/command/init.rb:41:in `validate!'

/Library/Ruby/Gems/2.3.0/gems/claide-1.0.2/lib/claide/command.rb:333:in `run'

/Library/Ruby/Gems/2.3.0/gems/cocoapods-1.3.1/lib/cocoapods/command.rb:52:in `run'

/Library/Ruby/Gems/2.3.0/gems/cocoapods-1.3.1/bin/pod:55:in `'

/usr/local/bin/pod:23:in `load'

/usr/local/bin/pod:23:in `'

```

――― TEMPLATE END ―――

'''

[!] 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

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Looking for related issues on cocoapods/cocoapods...

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and 37 more at:

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'''

可以看出是pod版本和xcode版本不兼容导致，所有要做pod升级

1.方案一 -- 失效

gem install cocoapods --pre

sudo gem install cocoapods --pre

2.方案二 -- 失效

gem update

sudo gem update

3.方案三 -- 成功

sudo gem update --system

sudo gem install -n /usr/local/bin cocoapods