Make .gemspec slimmer.

This is the same code that RubyGems are using:

a1a5ab55cb

[ci skip]
[Fix GH-1578]
Author:    Vít Ondruch <vondruch@redhat.com>

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@58304 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

tool/rbinstall.rb

@@ -784,7 +784,7 @@ def install_default_gem(dir, srcdir)
     puts "#{" "*30}#{gemspec.name} #{gemspec.version}"
     gemspec_path = File.join(default_spec_dir, "#{full_name}.gemspec")
     open_for_install(gemspec_path, $data_mode) do
-      gemspec.to_ruby
+      gemspec.to_ruby_for_cache
     end
 
     unless gemspec.executables.empty? then