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

https://github.com/ruby/forwardable/commit/1e7123a81b

lib/forwardable/forwardable.gemspec

@@ -20,7 +20,4 @@ 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"
 end