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/reader.h b/include/json/reader.h
index 959b5d6..caf8727 100644
--- a/include/json/reader.h
+++ b/include/json/reader.h
@@ -333,9 +333,9 @@
   Json::Value settings_;
 
   CharReaderBuilder();
-  ~CharReaderBuilder() override;
+  ~CharReaderBuilder() JSONCPP_OVERRIDE;
 
-  CharReader* newCharReader() const override;
+  CharReader* newCharReader() const JSONCPP_OVERRIDE;
 
   /** \return true if 'settings' are legal and consistent;
    *   otherwise, indicate bad settings via 'invalid'.