Compatible with PHP 5.3

Just a quick fix to the `SanitizerServiceProvider.php` file to use `$app` of `$this->app` inside a closure and be compatible with PHP 5.3.

```php
    public function register()
    {
        $this->app['sanitizer'] = $this->app->share(function ($app) {
            return new Sanitizer($app);
        });
    }
```