Change PHPDoc return type of values() (myclabs#37)

To improve IDE autocompletion on the resulting items when iterating

Author: holtkamp

src/Enum.php

@@ -86,7 +86,7 @@ public static function keys()
     /**
      * Returns instances of the Enum class of all Enum constants
      *
-     * @return array Constant name in key, Enum instance in value
+     * @return static[] Constant name in key, Enum instance in value
      */
     public static function values()
     {