This little code is more useful if you doesn't want keep old versions of installed gems

uninstall_old_gems.rb

`gem list`.split("\n").each do |line|
  next if line == ''
  name = line.scan(/^\S+/).first
  versions = line.scan(/\(([^\)]+)\)/).first.first.split(', ')
  versions.shift
  unless versions.empty?
    versions.each do |version|
      `gem uninstall #{name} -v #{version} --force`
    end
  end
end