tracking against chruby/auto.sh: chruby-fish defaults to system ruby when moving to a non-project directory

[todd-a-jacobs]

I'm opening this as a tracking item. Depending on the internals, this may get fixed along with any changes to /usr/local/share/chruby/auto.sh, but also may not if various functions have been rewritten directly in Fish. In either case, the way the default Ruby is stored and accessed will likely need to be Fish-specific due to the way Fish stores variables.

https://github.com/postmodern/chruby/issues/462