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!
Yeah, not sure why I doing it this way would work.

I think it's slightly more complicated than just checking that it's not `0.4.0` since fish support is actually available in `0.4.0`.  The user needs to be using a version that is either `> 0.4.0` (whenever that happens :smirk:) or a version with more commits since the exact point where fish support landed. Ie. where the commits since last tag `>= 56`.