rbinstall.rb: no post install messages

* tool/rbinstall.rb: suppress post install messages of rdoc for
  older ruby versions.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@50180 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

tool/rbinstall.rb

@@ -679,6 +679,12 @@ end
 end
 
 class Gem::Installer
+  install = instance_method(:install)
+  define_method(:install) do
+    spec.post_install_message = nil
+    install.bind(self).call
+  end
+
   generate_bin_script = instance_method(:generate_bin_script)
   define_method(:generate_bin_script) do |filename, bindir|
     generate_bin_script.bind(self).call(filename, bindir)