Cocoapods pod install 报错RuntimeError - [Xcodeproj] Unknown object version.

编辑好了Podfile，开始执行 pod install时报错，RuntimeError - [Xcodeproj] Unknown object version.
1、一顿搜索后原因诊断为cocoapods和Xcode版本不匹配，使用终端命令gem install cocoapods —pre执行。
2、但是报错
Fetching: cocoapods-core-1.5.3.gem (100%)
ERROR: While executing gem … (Gem::FilePermissionError)
You don’t have write permissions for the /Library/Ruby/Gems/2.3.0 directory.
3、搜到个方法执行终端输入命令 sudo chmod 777 /Library/Ruby/Gems/2.3.0
4、继续执行1中命令仍然报错
ERROR: While executing gem … (Errno::EACCES)
Permission denied @ rb_sysopen - /Library/Ruby/Gems/2.3.0/cache/cocoapods-core-1.5.3.gem
5、。。。。。。 sudo gem install 报错 ERROR: While executing gem … (Gem::CommandLineError)
Please specify at least one gem name (e.g. gem build GEMNAME)。。。。。。
6、最后碰到一个解决办法
sudo gem install -n /usr/local/bin cocoapods
成功后提示信息
Successfully installed cocoapods-1.5.3
Parsing documentation for cocoapods-1.5.3
Done installing documentation for cocoapods after 3 seconds
1 gem installed
7、再次执行 pod install，成功。
8、PS:应该是要从第1步是执行过后gem install cocoapods —pre，报错没权限后，直接执行sudo gem install -n /usr/local/bin cocoapods，应该就不用这么麻烦了，猜测。