Use Gemfile instead of Gem::Specification#add_development_dependency

Author: hsbt

Gemfile

@@ -1,6 +1,5 @@
 source "https://rubygems.org"
 
-git_source(:github) {|repo_name| "https://github.com/#{repo_name}" }
-
-# Specify your gem's dependencies in digest.gemspec
-gemspec
+gem "rake"
+gem "rake-compiler"
+gem "test-unit"

digest.gemspec

@@ -40,8 +40,4 @@ Gem::Specification.new do |spec|
     ext/digest/sha2/extconf.rb
   ]
   spec.metadata["msys2_mingw_dependencies"] = "openssl"
-
-  spec.add_development_dependency "bundler"
-  spec.add_development_dependency "rake"
-  spec.add_development_dependency "rake-compiler"
 end