added ORMPurger::PURGE_MODE_TRUNCATE as default purge mode for doctrine orm load fixtures

Added,

+      if ($type == 'ORM') {
+            if ($purgeMode === null) {
+                $purgeMode = ORMPurger::PURGE_MODE_TRUNCATE;
+            }
+        } 

Instead of adding it in method parameter,  it is added inside the method as ORMPurger class is not directly used, it is dynamically used like,

$purgerClass = 'Doctrine\\Common\\DataFixtures\\Purger\\'.$type.'Purger';