use: switch form PATH to fish_user_paths

This might impact a few people, but I think this approach is correct as it
prepends, but also gives you flexibility to manipulate this prepending part if
you don't want ruby path to be very first amongst these.

An upcoming idea is to allow users to configure if it should be prepended or
appended to the `fish_user_paths`. And also put in some bundler support with
binstubs.

Basically integrate this:

https://github.com/terlar/dotfiles/tree/f6a132761a25a5ccda429dafbf0fa1dc17376797/config/fish/plugins/bundler