fixes #30 rm second argument of EnumSet::attach

It will not be a bc break as the argument wasn't used and it was optional. If someone calls the method with a second argument PHP simply ignores it.

Author: Marc Bennewitz

src/MabeEnum/EnumSet.php

@@ -88,11 +88,10 @@ public function getFlags()
     /**
      * Attach a new enumeration or overwrite an existing one
      * @param Enum|scalar $enum
-     * @param mixed       $data
      * @return void
      * @throws InvalidArgumentException On an invalid given enum
      */
-    public function attach($enum, $data = null)
+    public function attach($enum)
     {
         $this->initEnum($enum);
         $ordinal = $enum->getOrdinal();