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

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.

This fixes `.rbenv-vars` projects files being read in Pow environment, where the primary Ruby process is `/usr/local/opt/pow/libexec/node_modules/nack/bin/nack_worker` and therefore the `ruby` shim initializes RBENV_DIR to `/usr/local/opt/pow/libexec/node_modules/nack/bin`, where no `.rbenv-vars` will be found. In Pow environment, though, $PWD matches the current project's dir.

Fixes #15