Mark NodeVisitorAbstract as abstract class

It doesn't actually have any abstract methods, but doesn't do
anything by itself and is intended for extension only.

Fixes #865.

lib/PhpParser/NodeVisitorAbstract.php

@@ -5,7 +5,7 @@ namespace PhpParser;
 /**
  * @codeCoverageIgnore
  */
-class NodeVisitorAbstract implements NodeVisitor
+abstract class NodeVisitorAbstract implements NodeVisitor
 {
     public function beforeTraverse(array $nodes) {
         return null;