安装 cocoapods 报错：
该问题主要是 镜像 的问题：
Mac:~ mac$ sudo gem install -n/usr/local/bin cocoapods
Password:
ERROR: Could not find a valid gem 'cocoapods' (>= 0), here is why:
Unable to download data from https://ruby.taobao.org/ - bad response Not Found 404 (https://gems.ruby-china.org/specs.4.8.gz)
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https://gems.ruby-china.org/ 访问不了
域名改为
https://gems.ruby-china.com/
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所以替换 ruby-china 提供镜像，正确的操作是：
$ gem sources --remove https://rubygems.org/
$ gem sources --remove https://ruby.taobao.org/ // 如果安装了淘宝的镜像,也删除
// 等有反应之后再敲入以下命令
$ gem sources -a https://gems.ruby-china.com/
为了验证你的Ruby镜像是并且仅是ruby-china，可以用以下命令查看：
$ gem sources -l
具体效果如下：
192:~ mac$ gem sources --remove https://rubygems.org/
source https://rubygems.org/ not present in cache
192:~ mac$ gem sources --remove https://ruby.taobao.org/
https://ruby.taobao.org/ removed from sources
192:~ mac$ gem sources -a https://gems.ruby-china.com/
https://gems.ruby-china.com/ added to sources
192:~ mac$ gem sources -l
*** CURRENT SOURCES ***
https://gems.ruby-china.com/
192:~ mac$ sudo gem update --system // 更新一下
Latest version already installed. Done.
$ sudo gem install -n /usr/local/bin cocoapods // 安装最新版本的 cocoapods
