replace rbenv with asdf-vm

[salzig]

hej andy,
saw you reference rbenv in your brewfile, you might like to hear about https://github.com/asdf-vm/asdf, as it's a rbenv replacement that also works for PHP, Erlang, NodeJS, Elixir, kubectl, helm, …. I use it for quite a while, works perfekt.

have a nice day :)