WP in subdir only works from the root dir

In #878 we introduced the ability to detect WP being installed in a subdirectory.

However, this only works if you run WP-CLI from the root dir.

Example:

```
$ ls
index.php wp wp-content wp-config.php
$ wp core version
3.9-alpha
$ cd wp-content
$ wp core version
Error: This does not seem to be a WordPress install.
```

This differs from the usual behavior, where `wp-cli.yml` and `wp-load.php` files are searched up the directory tree.