Prefer outer `autoload.php` file

When WP-CLI is installed as part of a larger project, it should ignore the local `./vendor/autoload.php` file.

The only reason that was not the case was because executing `vendor/bin/behat` was the recommended way of running the test suite, but that has changed since #665 landed.

The `./vendor/autoload.php` file is still useful when cloning directly from Github and we rely on it in the Travis CI script.