Use Gemfile instead of Gem::Specification#add_development_dependency.

Author: hsbt

Gemfile

@@ -1,4 +1,7 @@
 source "https://rubygems.org"
 
-# Specify your gem's dependencies in weakref.gemspec
 gemspec
+
+gem "bundler"
+gem "rake"
+gem "minitest"

weakref.gemspec

@@ -24,8 +24,4 @@ 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