Fix VS warnings

These don't really need to be const.

resolves open-source-parsers#369

Author: cdunn2001

include/json/value.h

@@ -43,7 +43,7 @@ class JSON_API Exception : public std::exception {
   ~Exception() throw() override;
   char const* what() const throw() override;
 protected:
-  std::string const msg_;
+  std::string msg_;
 };
 
 /** Exceptions which the user cannot easily avoid.