Fix build failure due to RubyGems
Fix build failure due to RubyGems

Use rubygems v2 because rubygems v3 drops support of Ruby 2.2 or former.
ref. https://github.com/rubygems/rubygems/blob/v3.0.0/rubygems-update.gemspec#L32

yasaichi/grease,yasaichi/grease,yasaichi/grease,yasaichi/grease