Update str_slug to Str::slug

str_slug was remove in Laravel 6

Author: jkudish

src/RouteToClass.php

@@ -2,6 +2,8 @@
 
 namespace Zschuessler\RouteToClass;
 
+use Illuminate\Support\Str;
+
 /**
  * Class RouteToClass
  *
@@ -125,15 +127,15 @@ public function addClass($value, $sanitizeClassString = true)
     /**
      * Sanitize CSS Class String
      *
-     * Currently a simple wrapper around the `str_slug` method in Laravel.
+     * Currently a simple wrapper around the `Str::slug` method in Laravel.
      *
      * @param $value string The class name to sanitize.
      *
      * @return string A sanitized class name (a valid css class, eg no special characters).
      */
     public function sanitizeClassString($value)
     {
-        return str_slug($value);
+        return Str::slug($value);
     }
 
     /**
@@ -177,4 +179,4 @@ public function generateClassString()
 
         return $classString;
     }
-}
+}