using arrays to specify rules in sometimes method of validation class

now we can use only string, for example:
```php
$v->sometimes(array('reason', 'cost'), 'required', function($input)
{
    return $input->games >= 100;
});
```
after this edition, we can use arrays to specify rules:
```php
$v->sometimes(array('reason', 'cost'), array('reason' => 'required', 'cost' => 'in:100,200'), function($input)
{
    return $input->games >= 100;
});
```