Auto merge of #2027 - hsbt:rename-rubygems, r=segiddins

Rename `-rubygems` option.

This option only needs Ruby 1.8. and It will remove at Ruby 2.5.

This commit was picked r60125 from ruby/ruby:  ruby/ruby@9de6c71

Author: bundlerbot

lib/rubygems/ext/rake_builder.rb

@@ -20,7 +20,7 @@ def self.build(extension, directory, dest_path, results, args=[], lib_dir=nil)
     rake = ENV['rake']
 
     rake ||= begin
-               "#{Gem.ruby} -rubygems #{Gem.bin_path('rake', 'rake')}"
+               "#{Gem.ruby} -rrubygems #{Gem.bin_path('rake', 'rake')}"
              rescue Gem::Exception
              end