如何强制gem升级到gem 1.3.1

我正在尝试安装这样的宝石:

C:\InstantRails\rails_apps\foodmarksthespot>ruby script/plugin install git://github.com/lazyatom/engines.git

返回此消息:

Rails requires RubyGems >= 1.3.1 (you have 1.2.0). Please `gem update --system` and try again.

但是当我尝试更新时使用:

gem update --system

它说:

Updating RubyGems
Nothing to update

这是在Windows上.如何强制它将系统升级到特定版本？

http://rubyforge.org/forum/forum.php?forum_id=28071

从那里:

NOTE: RubyGems 1.1 and 1.2 have problems upgrading when there is no
rubygems-update installed. You will need to follow the second set of update
instructions if you see "Nothing to update".

If you have an older version of RubyGems installed, then you can still
do it in two steps:

$ gem install rubygems-update (again, might need to be admin/root)
$ update_rubygems (... here too)