Allow access to app_dev.php whith PHP's dev webserver

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT
| Doc PR        |

If you use Vagrant, you can use PHP's built-in webserver like so:

```
app/console server:run 0.0.0.0:8000
```

With Vagrant port-forwarding you can access the application at http://localhost:8000/. The `0.0.0.0:8000` makes PHP listen to IP addresses other than 127.0.0.1.

However `app_dev.php` will forbid access to any IP other than `127.0.0.1`, which makes it unusable.

This is a simple fix to allow access to `app_dev.php` when running on PHP's built-in webserver.