Fall back to $PWD if no files found in $RBENV_DIR

This matches how `rbenv-version-file` searches for `.ruby-version`. $RBENV_DIR might be blank, or it might be a directory of an executable located outside of the current project. If any of this is true, no `.rbenv-vars` files will be found traversing it, so fall back to $PWD and try again.

Fixes #15