How to overcome error when updating sass via gem on Mac?

Question

macOS

Greetings.
I decided to update sass to the latest version and ran into this error:

$ sudo gem update sass

Updating installed gems
Updating sass
Building native extensions.  This could take a while...
ERROR:  Error installing sass:
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/ffi-1.9.18 for inspection.
Results logged to /Library/Ruby/Gems/2.0.0/gems/ffi-1.9.18/ext/ffi_c/gem_make.out
Nothing to update

I don’t understand Ruby ((
What can it be and how to overcome it?