Methods getLastPage() and getCurrentPage() should return integer

Methods getLastPage() and getCurrentPage() should return integer. This is atleast according to [ArraySerializer::paginator()](https://github.com/thephpleague/fractal/blob/master/src/Serializer/ArraySerializer.php#L76) where both are typecasted to integer. PR changes the docblock return values  from `string` to `integer`. 

```php
$currentPage = (int) $paginator->getCurrentPage();
$lastPage = (int) $paginator->getLastPage();

$pagination = array(
    'total' => (int) $paginator->getTotal(),
    'count' => (int) $paginator->getCount(),
    'per_page' => (int) $paginator->getPerPage(),
    'current_page' => $currentPage,
    'total_pages' => $lastPage,
);
```