doReplacements() added is_callable() verification

Thus, to extend the class, you can call methods replace*Rule*() dynamically on __call().

Ex:

```php
class CustomValidator extends \Illuminate\Validation\Validator
{
	// ...
	// ...	
    public function __call($method, $parameters)
    {
		// ...
		// ...	
        if(preg_match('/^replace/', $method)) {
            return $this->doMessageReplacement($parameters);
        }
		// ...
		// ...	

        parent::__call($method, $parameters);
    }
	
// ...
// ...
```