C++11: override keyword

Source : http://en.cppreference.com/w/cpp/language/override
diff --git a/include/json/value.h b/include/json/value.h
index 8e996ae..1ab22ef 100644
--- a/include/json/value.h
+++ b/include/json/value.h
@@ -40,8 +40,8 @@
 class JSON_API Exception : public std::exception {
 public:
   Exception(std::string const& msg);
-  virtual ~Exception() throw();
-  virtual char const* what() const throw();
+  ~Exception() throw() override;
+  char const* what() const throw() override;
 protected:
   std::string const msg_;
 };