Fix issue with rbenv plugin where some rbenv commands fail

Hey there!

rbenv officially supported fish on master some time ago. This just tweaks the rbenv plugin so that it enables shims properly. Previously, commands like `shell` and `rehash` would fail since the plugin was using a workaround for loading shims.

Thanks!
It's just a one-liner, actually.
```fish
» test $nonexistent; and echo true; or echo false
false

» set existent 1

» test $existent; and echo true; or echo false
true
```
I just wanted to make sure that `rbenv_dir` had a value.  I'll make this explicit, though.