安装流程：

1. 确认ruby是否安装

 $ruby -v
 ruby 2.0.0p648 (2015-12-16 revision 53162) [universal.x86_64-darwin15]

2. 更换gem源:http://rubygems-china.oss.aliyuncs.com

$gem sources -l
*** CURRENT SOURCES ***
https://rubygems.org/

$gem sources --remove https://rubygems.org/
https://rubygems.org/ removed from sources

$gem sources -a http://rubygems-china.oss.aliyuncs.com

3. 安装cocoapods

$sudo gem install cocoapods
...gems installed

下面这步很容易出错

pod setup

或者

pod search FMDB
Creating search index for spec repo 'master'.. Done!