* tool/downloader.rb (Downloader::RubyGems.download): verify gems only if RubyGems is 2.4 or later. old RubyGems fails to verify almost all of bundled gems. git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@55055 b2dd03c8-39d4-4d8f-98ff-823fe69b080e