[ruby/readline-ext] Use rake/extensiokntask to build

https://github.com/ruby/readline-ext/commit/b0b5f709bd

ext/readline/readline-ext.gemspec

@@ -19,4 +19,8 @@ Gem::Specification.new do |spec|
   spec.bindir        = "exe"
   spec.executables   = spec.files.grep(%r{^exe/}) { |f| File.basename(f) }
   spec.require_paths = ["lib"]
+
+  spec.add_development_dependency "bundler"
+  spec.add_development_dependency "rake"
+  spec.add_development_dependency "rake-compiler"
 end