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

https://github.com/ruby/weakref/commit/10d547ba12

lib/weakref/weakref.gemspec

@@ -29,8 +29,4 @@ 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", "~> 2.0"
-  spec.add_development_dependency "rake", "~> 10.0"
-  spec.add_development_dependency "minitest", "~> 5.0"
 end