安装顺序 ruby -->gem-->rails 安装 ruby 、gem都比较顺利，按照rails问题很多。
1.ERROR: Possible alternatives: rails
更换安装源 gem sources -a http://ruby.taobao.org/：
增加新镜像 gem sources -r https://rubygems.org/：
移除原有镜像 gem sources -l；查看镜像列表（必须确保只有淘宝一个镜像地址）
2. ERROR: Error installing rails:
ERROR: Failed to build gem native extension. /usr/bin/ruby2.0 extconf.rb mkmf.rb can’t find header files for ruby at /usr/lib/ruby/include/ruby.h
安装ruby2.1-dev
sudo apt-get install ruby2.1-dev
3.ERROR: Error installing rails:
ERROR: Failed to build gem native extension. Gem files will remain installed in /var/lib/gems/2.1.0/gems/nokogiri-1.6.6.2 for inspection.
安装 libpq-dev
sudo apt-get install libpq-dev
4.sqlite error
apt-get install libsqlite3-dev