Use Gemfile instead of Gem::Specification#add_development_dependency.

Author: hsbt

Gemfile

@@ -3,3 +3,9 @@ source "https://rubygems.org"
 git_source(:github) {|repo_name| "https://github.com/#{repo_name}" }
 
 gemspec
+
+group :development do
+  gem "bundler"
+  gem "rake"
+  gem "test-unit"
+end

forwardable.gemspec

@@ -20,7 +20,4 @@ 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"
-  spec.add_development_dependency "rake"
 end