Overloaded std::ostream operators for enum types are not being used

[vitaut]

From @Razorx1989:

If someone tries to implement output operators for enum types (for much more informative output) then spdlog just implicitly casts the enum to int and gives the operator no chance. This can be fixed by changing the IsConvertibleToInt type trait.

enum { value = !std::is_enum::value && (sizeof(convert(get())) == sizeof(yes)) };

[vitaut]

Fixed in 6cff6d8 after some struggle with clang =).