Use Gemfile instead of Gem::Specification#add_development_dependency.

Author: hsbt

Gemfile

@@ -1,4 +1,10 @@
 source 'https://rubygems.org'
 
-# Specify your gem's dependencies in gdbm.gemspec
 gemspec
+
+group :development do
+  gem "bundler"
+  gem "rake"
+  gem "rake-compiler"
+  gem "test-unit"
+end

gdbm.gemspec

@@ -18,9 +18,4 @@ Gem::Specification.new do |spec|
   spec.require_paths = ["lib"]
   spec.extensions    = ["ext/gdbm/extconf.rb"]
   spec.required_ruby_version = ">= 2.3.0"
-
-  spec.add_development_dependency "bundler", "~> 1.14"
-  spec.add_development_dependency "rake", "~> 10.0"
-  spec.add_development_dependency "rake-compiler"
-  spec.add_development_dependency "test-unit"
 end