Unset RUBY_VERSION_FILE when loading auto.sh

Ensure that chruby will be called or reseted after auto.sh is loaded.
This fixes a problem when for example a new tmux window is opened and
environment variables (including RUBY_*) reserved, but PATH is reset in
.zshrc/.bashrc. The root cause is thus same than in #46.
Merge pull request #83 from tmatilai/unset-version-file

Unset RUBY_VERSION_FILE when loading auto.sh