bump nokogiri to ~> 1.8.1

Author: kivikakk

github-markup.gemspec

@@ -21,7 +21,7 @@ Gem::Specification.new do |s|
   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 'nokogiri', '1.6.8.1'
+  s.add_development_dependency 'nokogiri', '~> 1.8.1'
   s.add_development_dependency 'nokogiri-diff', '~> 0.2.0'
   s.add_development_dependency "github-linguist", "~> 6.0"
 end