Use required_rubygems_version
Use required_rubygems_version

Correct `rubygems_version` to `required_rubygems_version`,
because `rubygems_version` should not be set.
refs: https://github.com/rubygems/rubygems/blob/cc6f96aa0c1c99d94363d12884001a2de28da033/lib/rubygems/specification.rb#L530-L534

cucumber/cucumber-rails,cucumber/cucumber-rails,cucumber/cucumber-rails