Namespaced pool should implement HierarchicalPool

Fix Prefixed pool property annotation
Fix #105

Author: gsouf

NamespacedCachePool.php

@@ -13,15 +13,14 @@
 
 use Cache\Hierarchy\HierarchicalPoolInterface;
 use Psr\Cache\CacheItemInterface;
-use Psr\Cache\CacheItemPoolInterface;
 
 /**
  * Prefix all the stored items with a namespace. Also make sure you can clear all items
  * in that namespace.
  *
  * @author Tobias Nyholm <tobias.nyholm@gmail.com>
  */
-class NamespacedCachePool implements CacheItemPoolInterface
+class NamespacedCachePool implements HierarchicalPoolInterface
 {
     /**
      * @type HierarchicalPoolInterface