COMP:  Use C++11 override directly

The override support in C++11 is required so avoid aliasing
this feature.  Compilers that do not support the override keyword
are no longer supported.
diff --git a/include/json/value.h b/include/json/value.h
index 3d765ec..3fdd884 100644
--- a/include/json/value.h
+++ b/include/json/value.h
@@ -55,8 +55,8 @@
 class JSON_API Exception : public std::exception {
 public:
   Exception(JSONCPP_STRING const& msg);
-  ~Exception() JSONCPP_NOEXCEPT JSONCPP_OVERRIDE;
-  char const* what() const JSONCPP_NOEXCEPT JSONCPP_OVERRIDE;
+  ~Exception() JSONCPP_NOEXCEPT override;
+  char const* what() const JSONCPP_NOEXCEPT override;
 
 protected:
   JSONCPP_STRING msg_;