Fix issue where chruby_auto is always executed

When sourcing `auto.sh`, the first line unsets the RUBY_AUTO_VERSION variable.
This causes `chruby_auto` to always detect an "incorrect" active ruby, even if
current ruby equals ruby requested by .ruby-version.

By unsetting `BASH_VERSION`, the initial `chruby_auto` isn't triggered,
allowing us to set it first, and then manually triggering the `chruby_auto`
command.

closes #9