Use Gemfile instead of Gem::Specification#add_development_dependency

Author: hsbt

Gemfile

@@ -12,3 +12,6 @@ when 'ext', nil
 when 'pure'
   gemspec :name => 'json_pure'
 end
+
+gem "rake"
+gem "test-unit"

json-java.gemspec

@@ -25,9 +25,6 @@ spec = Gem::Specification.new do |s|
   }
 
   s.required_ruby_version = Gem::Requirement.new(">= 2.3")
-
-  s.add_development_dependency "rake"
-  s.add_development_dependency "test-unit"
 end
 
 if $0 == __FILE__

json.gemspec

@@ -64,7 +64,4 @@ Gem::Specification.new do |s|
   }
 
   s.required_ruby_version = Gem::Requirement.new(">= 2.3")
-
-  s.add_development_dependency "rake"
-  s.add_development_dependency "test-unit"
 end

json_pure.gemspec

@@ -52,7 +52,4 @@ Gem::Specification.new do |s|
   }
 
   s.required_ruby_version = Gem::Requirement.new(">= 2.3".freeze)
-
-  s.add_development_dependency "rake"
-  s.add_development_dependency "test-unit"
 end