ERROR: Could not find a valid gem 'rails' (>= 0), here is why:
Unable to download data from https://rubygems.org/ - Errno::ETIMEDOUT: Connection timed out - connect(2) for "s3.amazonaws.com" port 443 (https://api.rubygems.org/latest_specs.4.8.gz)
后台在Google上找到解决方案：
sudo gem sources -r https://rubygems.org
sudo gem sources -a http://rubygems.org 貌似意思是将https替换成http，再试了一下，果然可以了，但是安装完后别忘了把它设回来：
sudo gem sources -r http://rubygems.org
sudo gem sources -a https://rubygems.org
解决方案参考来源：
http://stackoverflow.com/a/19179835/1227911
------------------------ 更新 ---------------------------
虽然用了以上的方法，但是还是时不时的抽风，这要归功于天朝的网络了，现在还有另外的一个方案，就是使用国内淘宝网提供的RubyGems镜像作为源：
sudo gem sources -r https://rubygems.org/
sudo gem sources -a http://ruby.taobao.org/