Specify require_ruby_version, drops 2.3.x

Author: satoryu

docx.gemspec

@@ -11,6 +11,7 @@ Gem::Specification.new do |s|
   s.email       = ['chrahunt@gmail.com']
   s.homepage    = 'https://github.com/chrahunt/docx'
   s.files       = Dir["README.md", "LICENSE.md", "lib/**/*.rb"]
+  s.required_ruby_version = '>= 2.4.6'
 
   s.add_dependency 'nokogiri', '~> 1.8', '>= 1.8.1'
   s.add_dependency 'rubyzip',  '~> 2.0'