Use required_rubygems_version instead of rubygems_version (#1629)

* Use required_rubygems_version

Use `required_rubygems_version` instead of `rubygems_version`,
because `rubygems_version` should not be set.
refs: https://github.com/rubygems/rubygems/blob/2faada63ae65d8559bcba9e083f3c35ba6c18fdc/lib/rubygems/specification.rb#L536-L541

* Update CHANGELOG.md

Author: mishina2228

CHANGELOG.md

@@ -24,6 +24,8 @@ Please visit [cucumber/CONTRIBUTING.md](https://github.com/cucumber/cucumber/blo
 
 ### Fixed
 
+- Use `required_rubygems_version` instead of `rubygems_version`([PR#1629](https://github.com/cucumber/cucumber-ruby/pull/1629))
+
 ### Changed
 
 ### Removed

cucumber.gemspec

@@ -49,8 +49,8 @@ Gem::Specification.new do |s|
   s.add_development_dependency 'rack-test', '~> 1.1', '>= 1.1.0'
   s.add_development_dependency 'sinatra', '~> 2.1', '>= 2.1.0'
 
-  s.rubygems_version = '>= 1.6.1'
-  s.files            = Dir[
+  s.required_rubygems_version = '>= 1.6.1'
+  s.files = Dir[
     'CHANGELOG.md',
     'CONTRIBUTING.md',
     'README.md',