1. how I installed Sass on Ubuntu 16.04
  2. $ sudo apt-get install ruby
    $ ... 
    $ sudo gem install sass --no-user-install
    … after the second of the above invocations I got the following:
    Fetching: rb-fsevent-0.10.2.gem (100%)
    Successfully installed rb-fsevent-0.10.2
    Fetching: ffi-1.9.18.gem (100%)
    Building native extensions.  This could take a while...
    ERROR:  Error installing sass:
    ERROR: Failed to build gem native extension.
    
        current directory: /var/lib/gems/2.3.0/gems/ffi-1.9.18/ext/ffi_c
    /usr/bin/ruby2.3 -r ./siteconf20170913-1057-1vplk2n.rb extconf.rb
    mkmf.rb can't find header files for ruby at /usr/lib/ruby/include/ruby.h
    
    extconf failed, exit code 1
    
    Gem files will remain installed in /var/lib/gems/2.3.0/gems/ffi-1.9.18 for inspection.
    Results logged to /var/lib/gems/2.3.0/extensions/x86_64-linux/2.3.0/ffi-1.9.18/gem_make.out
          
    … following the advice given here I then did:
    sudo apt-get install ruby-dev
    … at which point the
    sudo gem install sass --no-user-install
    succeeded.