[ruby/rdoc] Move dev dependency of gettext to Gemfile

https://github.com/ruby/rdoc/commit/a177377b6f

lib/rdoc/Gemfile

@@ -9,4 +9,5 @@ group :development do
   gem 'test-unit'
   gem 'minitest' # for test_rdoc_rubygems_hook.rb
   gem 'rubocop'
+  gem 'gettext'
 end

lib/rdoc/rdoc.gemspec

@@ -246,5 +246,4 @@ RDoc includes the +rdoc+ and +ri+ tools for generating and displaying documentat
   s.required_rubygems_version = Gem::Requirement.new(">= 2.2")
 
   s.add_dependency 'psych', '>= 4.0.0'
-  s.add_development_dependency("gettext")
 end