Updated method of creating new DateTime to work with the clockmock (#55)

Author: Nyholm

CacheItem.php

@@ -154,7 +154,7 @@ public function expiresAfter($time)
         }
 
         if (is_int($time)) {
-            $this->expirationDate = new \DateTime(sprintf('+%sseconds', $time));
+            $this->expirationDate = \DateTime::createFromFormat('U', time() + $time);
         }
 
         return $this;