Redeclare $_attributes property in Configuration class that has b…

…een removed in doctrine/dbal:3.x

To keep backward compatibility we need to redeclare this property to keep using it.
Partially fixes doctrine#8884

lib/Doctrine/ORM/Configuration.php

@@ -54,6 +54,9 @@
  */
 class Configuration extends \Doctrine\DBAL\Configuration
 {
+    /** @var mixed[] */
+    protected $_attributes = []; /** @codingStandardsIgnoreLine */
+
     /**
      * Sets the directory where Doctrine generates any necessary proxy class files.
      *