racc and mini_portile2 are needed only to build native extension.

--- a/nokogiri.gemspec
+++ b/nokogiri.gemspec
@@ -339,9 +339,9 @@
     spec.requirements << "jar xerces, xercesImpl, 2.12.2" # https://search.maven.org/artifact/xerces/xercesImpl
     spec.requirements << "jar xml-apis, xml-apis, 1.4.01" # https://search.maven.org/artifact/xml-apis/xml-apis
   else
-    spec.add_runtime_dependency("mini_portile2", "~> 2.8.2") # keep version in sync with extconf.rb
+    spec.add_development_dependency("mini_portile2", "~> 2.8.2") # keep version in sync with extconf.rb
   end
-  spec.add_runtime_dependency("racc", "~> 1.4")
+  spec.add_development_dependency("racc", "~> 1.4")
 
   spec.extensions << "ext/nokogiri/extconf.rb"
 end