安装最新版cocoapods

gem 的sources源经常变化，
在sources源不正确的情况下，目前可能报这样的错误:

bogon:~ wuhaiwei$ gem sources -a https://gems.ruby-china.org/
Error fetching https://gems.ruby-china.org/:
    bad response Not Found 404 (https://gems.ruby-china.org/specs.4.8.gz)

确保sources源修改为最新地址: https://gems.ruby-china.com/

bogon:~ wuhaiwei$ gem sources -l
*** CURRENT SOURCES ***

https://gems.ruby-china.com/

//注意cocoapods是严格大小写的
bogon:~ wuhaiwei$ gem install cocoapods

世界终于安静...