Use *_t and *_v trait helpers from C++17 STL (#731)

include/mbgl/actor/actor_ref.hpp

@@ -38,11 +38,7 @@ public:
     template <typename Fn, class... Args>
     auto ask(Fn fn, Args&&... args) const {
         // Result type is deduced from the function's return type
-#if __cplusplus >= 201703L
         using ResultType = std::invoke_result_t<decltype(fn), Object, Args...>;
-#else
-        using ResultType = typename std::result_of<decltype(fn)(Object, Args...)>::type;
-#endif
 
         std::promise<ResultType> promise;
         auto future = promise.get_future();