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
Merge pull request #16 from mislav/pwd-fallback

Fall back to $PWD if no vars found in $RBENV_DIR; fixes Pow