postmodern/chruby — pull request #243 by aprescott (2014-01-29 01:12:46+00:00)
Title: 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.