[ruby/stringio] Use Gemfile instead of Gem::Specification#add_development_dependency.

https://github.com/ruby/stringio/commit/157fe9f04d

ext/stringio/stringio.gemspec

@@ -28,7 +28,4 @@ Gem::Specification.new do |s|
 
   # s.cert_chain  = %w[certs/nobu.pem]
   # s.signing_key = File.expand_path("~/.ssh/gem-private_key.pem") if $0 =~ /gem\z/
-
-  s.add_development_dependency 'rake-compiler'
-  s.add_development_dependency 'test-unit'
 end