Error installing watir

[Martin Delille]

When I try to install watir I have the following error:

$ gem install watir
Building native extensions.  This could take a while...
ERROR:  Error installing safariwatir:
	ERROR: Failed to build gem native extension.

    current directory: /usr/local/lib/ruby/gems/2.4.0/gems/rb-appscript-0.6.1
/usr/local/opt/ruby/bin/ruby -r ./siteconf20170123-96895-5xx3by.rb extconf.rb
*** extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of necessary
libraries and/or headers.  Check the mkmf.log file for more details.  You may
need configuration options.

Provided configuration options:
	--with-opt-dir
	--with-opt-include
	--without-opt-include=${opt-dir}/include
	--with-opt-lib
	--without-opt-lib=${opt-dir}/lib
	--with-make-prog
	--without-make-prog
	--srcdir=.
	--curdir
	--ruby=/usr/local/Cellar/ruby/2.4.0/bin/$(RUBY_BASE_NAME)
extconf.rb:44:in `<main>': uninitialized constant Config (NameError)
Did you mean?  RbConfig
               CONFIG

extconf failed, exit code 1

Gem files will remain installed in /usr/local/lib/ruby/gems/2.4.0/gems/rb-appscript-0.6.1 for inspection.
Results logged to /usr/local/lib/ruby/gems/2.4.0/extensions/x86_64-darwin-15/2.4.0/rb-appscript-0.6.1/gem_make.out

My configuration:

• MacOS 10.11.6
• Ruby 2.4.0p0
• Gem 2.6.8