script/plugin:3:in `require': no such file to load -- commands/plugin

When I try and run `script/plugin install git://github.com/rails/rails_upgrade.git` I get this:

```
script/plugin:3:in `require': no such file to load -- commands/plugin (LoadError)
    from script/plugin:3
```

Not sure that to do with this. Any thoughts? Thanks!
