Auto-detect composer installed CakePHP folder.

It feels a little bit awkward having to adjust index.php and test.php manually after baking a new repo.
And especially having to add complete lines to make composer work out of the box.

This patch makes the auto-detection work without any modifications necessary.
Isn't that just the normal way when using composer and a 2.x app? It also aligns with the documentation we have right now (advanced installation section).
```
- app
    - Vendor (composer vendor path)
    - Plugin (composer plugin path)
    - ...
```

It also just auto-detects. You can also change it again to whatever you like.
The point is that if you start using 2.x and composer you should not have to do that - especially as beginner.