Depends on chruby-fish: https://github.com/JeanMertz/chruby-fish
set CHRUBY_AUTO_ENABLED to false to disable auto loading Ruby versions on
directory change.
set CHRUBY_ROOT to point to the root path of chruby. The path will be
appended by share/chruby/chruby.fish and share/chruby/auto.fish.