Fix ambiguous function template overload in json.hpp

There are already the "small signed/unsigned overloads" of loadValue that
are always selected for integers smaller than 64 bits.

The fixed overloads conflicted with the "small signed/unsigned overloads"
if:

- long is smaller than 64 bits
- int32_t is not typedef for long

include/cereal/archives/json.hpp

@@ -641,14 +641,14 @@ namespace cereal
       //! Serialize a long if it would not be caught otherwise
       template <class T> inline
       typename std::enable_if<std::is_same<T, long>::value &&
-                              !std::is_same<T, std::int32_t>::value &&
+                              sizeof(T) >= sizeof(std::int64_t) &&
                               !std::is_same<T, std::int64_t>::value, void>::type
       loadValue( T & t ){ loadLong(t); }
 
       //! Serialize an unsigned long if it would not be caught otherwise
       template <class T> inline
       typename std::enable_if<std::is_same<T, unsigned long>::value &&
-                              !std::is_same<T, std::uint32_t>::value &&
+                              sizeof(T) >= sizeof(std::uint64_t) &&
                               !std::is_same<T, std::uint64_t>::value, void>::type
       loadValue( T & t ){ loadLong(t); }
       #endif // _MSC_VER