bug fix : delete mementos with public virtual destructor

Author: mynickmynick

src/Memento/Conceptual/main.cc

@@ -27,6 +27,7 @@
  */
 class Memento {
  public:
+  virtual ~Memento() {}
   virtual std::string GetName() const = 0;
   virtual std::string date() const = 0;
   virtual std::string state() const = 0;