Use Gemfile instead of Gem::Specification#add_development_dependency.

.gitignore

@@ -6,3 +6,4 @@
 /pkg/
 /spec/reports/
 /tmp/
+Gemfile.lock

Gemfile

@@ -1,4 +1,9 @@
 source "https://rubygems.org"
 
-# Specify your gem's dependencies in net-smtp.gemspec
 gemspec
+
+group :development do
+  gem "bundler"
+  gem "rake"
+  gem "test-unit"
+end

Rakefile

@@ -4,7 +4,7 @@ require "rake/testtask"
 Rake::TestTask.new(:test) do |t|
   t.libs << "test"
   t.libs << "lib"
-  t.test_files = FileList["test/**/*_test.rb"]
+  t.test_files = FileList['test/**/test_*.rb']
 end
 
-task :default => :test
+task :default => [:test]

net-smtp.gemspec

@@ -8,26 +8,17 @@ Gem::Specification.new do |spec|
   spec.authors       = ["Hiroshi SHIBATA"]
   spec.email         = ["hsbt@ruby-lang.org"]
 
-  spec.summary       = %q{TODO: Write a short summary, because RubyGems requires one.}
-  spec.description   = %q{TODO: Write a longer description or delete this line.}
-  spec.homepage      = "TODO: Put your gem's website or public repo URL here."
-
-  spec.metadata["allowed_push_host"] = "TODO: Set to 'http://mygemserver.com'"
+  spec.summary       = %q{Simple Mail Transfer Protocol client library for Ruby.}
+  spec.description   = %q{Simple Mail Transfer Protocol client library for Ruby.}
+  spec.homepage      = "https://github.com/ruby/net-smtp"
 
   spec.metadata["homepage_uri"] = spec.homepage
-  spec.metadata["source_code_uri"] = "TODO: Put your gem's public repo URL here."
-  spec.metadata["changelog_uri"] = "TODO: Put your gem's CHANGELOG.md URL here."
+  spec.metadata["source_code_uri"] = spec.homepage
 
-  # Specify which files should be added to the gem when it is released.
-  # The `git ls-files -z` loads the files in the RubyGem that have been added into git.
   spec.files         = Dir.chdir(File.expand_path('..', __FILE__)) do
     `git ls-files -z`.split("\x0").reject { |f| f.match(%r{^(test|spec|features)/}) }
   end
   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