racc and mini_portile2 are needed only to build native extension.

--- a/nokogiri.gemspec
+++ b/nokogiri.gemspec
@@ -318,8 +318,8 @@
   spec.extra_rdoc_files += Dir.glob("README.md")
   spec.rdoc_options = ["--main", "README.md"]

-  spec.add_runtime_dependency("mini_portile2", "~> 2.8.0") unless java_p # keep version in sync with extconf.rb
-  spec.add_runtime_dependency("racc", "~> 1.4")
+  spec.add_development_dependency("mini_portile2", "~> 2.8.0") unless java_p # keep version in sync with extconf.rb
+  spec.add_development_dependency("racc", "~> 1.4")

   spec.add_development_dependency("bundler", "~> 2.2")
   spec.add_development_dependency("hoe-markdown", "~> 1.4")