Fix an MSVC warning (#798)

fmt/format.h

@@ -1287,6 +1287,7 @@ struct ConvertToIntImpl2<T, true> {
 template <typename T>
 struct ConvertToInt {
   enum {
+#pragma warning(suppress: 4244)
     enable_conversion = sizeof(fmt::internal::convert(get<T>())) == sizeof(Yes)
   };
   enum { value = ConvertToIntImpl2<T, enable_conversion>::value };

fmt/ostream.h

@@ -64,6 +64,7 @@ template <typename T>
 struct ConvertToIntImpl<T, true> {
   // Convert to int only if T doesn't have an overloaded operator<<.
   enum {
+#pragma warning(suppress: 4244)
     value = sizeof(convert(get<DummyStream>() << get<T>())) == sizeof(No)
   };
 };