How to add a (pre/post)_install_hook to ruby gems

I want to execute some code at pre install time when installing a gem from rubygems.org with a command like gem install some-gem.

The documentation states that you can use a http://docs.ruby-lang.org/en/2.2.0/Gem.html pre_install hook that looks like this:

# File lib/rubygems.rb, line 724
def self.pre_install(&hook)
  @pre_install_hooks << hook
end

The doc further states:

RubyGems defaults are stored in rubygems/defaults.rb. If you're packaging RubyGems or implementing Ruby you can change RubyGems' defaults. For RubyGems packagers, provide lib/rubygems/defaults/operating_system.rb and override any defaults from lib/rubygems/defaults.rb. For Ruby implementers, provide lib/rubygems/defaults/#{RUBY_ENGINE}.rb and override any defaults from lib/rubygems/defaults.rb. If you need RubyGems to perform extra work on install or uninstall, your defaults override file can set pre and post install and uninstall hooks. See ::pre_install, ::pre_uninstall, ::post_install, ::post_uninstall.

This sounds exactly what I want. So I created the files

• lib/rubygems/defaults/defaults.rb
• lib/rubygems/defaults/operating_system.rb
• rubygems/defaults.rb

and I put the code

Gem.pre_install { puts 'pre install hook called!' }

in all of the listed files. Then I added them to require_paths in the gemspec like this:

s.require_paths = ["lib", "test", "rubygems"]

But I see no output in the terminal when installing the gem.

What am I doing wrong?