Enable swap memory

Those who use a lower memory limit for homestead have been running into the error below when executing `composer update` or `composer install`:

```
Fatal error: Uncaught exception 'ErrorException' with message 'fork failed - Cannot allocate memory' in phar://composer.phar/bin/../src/../vendor/symfony/console/Symfony/Component/Console/Application.php:983
```

Creating a swap memory solves that. Thanks [Mark Vaughn](https://coderwall.com/p/grhteq)!