Support 'chruby -' to switch to previous version

From a discussion at #242, this implements `chruby -` to switch to the previous version.

```
$ chruby system # system Ruby
$ chruby 2.0    # 2.0
$ chruby -      # system
$ chruby -      # 2.0
$ chruby 2.1    # 2.1
$ chruby -      # 2.0
```

I realise that the contributing guidelines say don't add further environment variables, but @postmodern thought `chruby -` was a good idea and I don't think there's a better way to implement this.