centos 使用 jekyll 安装ruby 使用 gem 命令
参考：https://www.digitalocean.com/community/articles/how-to-install-ruby-2-1-0-on-centos-6-5-using-rvm
http://jekyllrb.com/
install rvm
install ruby
use gem
- update RVM:
rvm get head - read and follow instructions:
rvm requirements - reinstall ruby:
rvm reinstall 1.9.2
