Use macro for `override`

b/c MS VS2010 is supposed to be C++11 but does not fulfull
the entire standard.

Resolves #410.
Re: #430.
diff --git a/include/json/value.h b/include/json/value.h
index 20747fe..4c29382 100644
--- a/include/json/value.h
+++ b/include/json/value.h
@@ -53,8 +53,8 @@
 class JSON_API Exception : public std::exception {
 public:
   Exception(JSONCPP_STRING const& msg);
-  ~Exception() throw() override;
-  char const* what() const throw() override;
+  ~Exception() throw() JSONCPP_OVERRIDE;
+  char const* what() const throw() JSONCPP_OVERRIDE;
 protected:
   JSONCPP_STRING msg_;
 };