code reformatting
unset RUBY_AUTO_VERSION when loading auto.fish
Abort `chruby_use` if chruby-bash returns non-zero exit status

In the old situation, `bchruby` was triggered before `chruby_use` to verify its
exit-code. This resulted in a minimum of two (and possibly three, when
triggered through `auto.fish`) calls to `bchruby`, causing significant
slowdowns.