Bump version of required_ruby_version to 2.3. Fixes #464

ext/json/json.gemspec

@@ -65,7 +65,7 @@ Gem::Specification.new do |s|
     'wiki_uri'          => 'https://github.com/flori/json/wiki'
   }
 
-  s.required_ruby_version = Gem::Requirement.new(">= 2.0")
+  s.required_ruby_version = Gem::Requirement.new(">= 2.3")
   s.test_files = ["tests/test_helper.rb"]
 
   s.add_development_dependency "rake"