当执行gem install rails时报错，报错信息：
Building native extensions. This could take a while...
ERROR: Error installing rails:
ERROR: Failed to build gem native extension.
/System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/bin/ruby extconf.rb
mkmf.rb can't find header files for ruby at /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/include/ruby.h
Gem files will remain installed in /Library/Ruby/Gems/2.0.0/gems/atomic-1.1.14 for inspection
Results logged to /Library/Ruby/Gems/2.0.0/gems/atomic-1.1.14/ext/gem_make.out
解决办法：
1.gem update --system
2.sudo apt-get install ruby-dev