avoids php blowing up if an array value is passed

if incoming data has an array value set with a slug rule on it, you get 
preg_match() expects parameter 2 to be string, array given
vendor\vlucas\valitron\src\Valitron\Validator.php:571

src/Valitron/Validator.php

@@ -565,6 +565,9 @@ class Validator
      */
     protected function validateSlug($field, $value)
     {
+        if(is_array($value)) {
+            return false;
+        }
         return preg_match('/^([-a-z0-9_-])+$/i', $value);
     }