`rbenv shell -` allows you to switch to the previously activated ruby
version. Similar to `cd -` or `git checkout -`.
This tries to implement `rbenv shell -` as proposed in #854. However,
adding support seemed to break the "shell change version" test. I'm not
very good at Bash programming, can someone tell me what is wrong with
what I'm doing? I'd like to add a bit more functionality to this, but
I'm really just cargo cult programming Bash.
Thank you!
fix tests
6a912bf104