Can't install gem: Failed to build gem native extension while trying to install Watir

Question

I am trying to install Watir, but when I try to run sudo gem install watir I get this error

Building native extensions.  This could take a while...
ERROR:  Error installing watir:
    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 ./siteconf20170429-29912-10tswho.rb extconf.rb
mkmf.rb can't find header files for ruby at /usr/lib/ruby/include/ruby.h

How can I install Watir?

Solution

I had to run a few commands to get it to work.

sudo apt-get install ruby-dev

Then, while looking back at the documentation, I realized I also had to install selenium-webdriver

sudo gem install selenium-webdriver

You might also have to run the following command before installing selenium-webdriver

sudo gem update --system