localhost:~ liuliuxin$ sudo gem install /Users/liuliuxin/Downloads/sass-3.6.0.gem 

Password:

Building native extensions. This could take a while...

ERROR:  Error installing /Users/liuliuxin/Downloads/sass-3.6.0.gem:

ERROR: Failed to build gem native extension.

    current directory: /Library/Ruby/Gems/2.3.0/gems/ffi-1.9.25/ext/ffi_c

/System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/bin/ruby -r ./siteconf20181011-8740-dsf802.rb extconf.rb

mkmf.rb can't find header files for ruby at /System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/lib/ruby/include/ruby.h

extconf failed, exit code 1

Gem files will remain installed in /Library/Ruby/Gems/2.3.0/gems/ffi-1.9.25 for inspection.

Results logged to /Library/Ruby/Gems/2.3.0/extensions/universal-darwin-17/2.3.0/ffi-1.9.25/gem_make.out