RVM has updated the gemset selector to +

In RVM head as of today, you now have to do eg

```
rvm use 1.9.1+aruba
```

Instead of

```
rvm use 1.9.1%aruba
```

I can only find the announcement on Twitter: http://twitter.com/wayneeseguin/status/10848934147

As I've just updated to RVM head I'll create a micro-patch for this
