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!
Should we check if version is *not* 0.4.0 instead? So we know they are using rbenv from git?

````
.oh-my-fish:master λ rbenv --version
rbenv 0.4.0
````