Updated Yard to 0.9.20 to avoid the two vulnerabilities reported by dependabot.

Author: evolve75

rubytree.gemspec

@@ -83,7 +83,7 @@ Gem::Specification.new do |s|
   s.add_development_dependency 'rubocop-rake', '~> 0.0'
   s.add_development_dependency 'rubocop-rspec', '~> 2.0'
   s.add_development_dependency 'test-unit', '~> 3.0'
-  s.add_development_dependency 'yard', '~> 0.0', '> 0.9'
+  s.add_development_dependency 'yard', '~> 0.0', '>= 0.9.20'
 
   s.post_install_message = <<-END_MESSAGE
     ========================================================================