Fix compilation with clang 4.0 in -std=c++1z mode

Author: dean0x7d

include/pybind11/common.h

@@ -427,12 +427,8 @@ template<size_t N> using make_index_sequence = typename make_index_sequence_impl
 template <bool B> using bool_constant = std::integral_constant<bool, B>;
 template <typename T> struct negation : bool_constant<!T::value> { };
 
-#ifdef __cpp_lib_void_t
-using std::void_t;
-#else
 template <typename...> struct void_t_impl { using type = void; };
 template <typename... Ts> using void_t = typename void_t_impl<Ts...>::type;
-#endif
 
 /// Compile-time all/any/none of that check the boolean value of all template types
 #ifdef __cpp_fold_expressions