use modern sanitize in tests

(Vulnerability alert doesn't exactly apply as it's never used in actual
execution, but that's okay.)

Author: kivikakk

github-markup.gemspec

@@ -21,7 +21,7 @@ Gem::Specification.new do |s|
   s.add_development_dependency 'activesupport', '~> 4.0'
   s.add_development_dependency 'minitest', '~> 5.4', '>= 5.4.3'
   s.add_development_dependency 'html-pipeline', '~> 1.0'
-  s.add_development_dependency 'sanitize', '~> 2.1', '>= 2.1.0'
+  s.add_development_dependency 'sanitize', '>= 4.6.3'
   s.add_development_dependency 'nokogiri', '~> 1.8.1'
   s.add_development_dependency 'nokogiri-diff', '~> 0.2.0'
   s.add_development_dependency "github-linguist", ">= 7.1.3"