message: Make changes on the make_unsigned type trait
  Fix a typo in the static assert message in `make_unsigned`
  Support more specifications for `make_unsigned`
diff: @@ -148,11 +148,15 @@ inline void vector_emplace_back(std::vector<T> *vector, V &&data) {
  template <typename T> using is_unsigned = std::tr1::is_unsigned<T>;
  // Android NDK doesn't have std::make_unsigned or std::tr1::make_unsigned.
  template<typename T> struct make_unsigned {
  - static_assert(is_unsigned<T>::value, "Specialization not impelented!");
  + static_assert(is_unsigned<T>::value, "Specialization not implemented!");
  using type = T;
  };
  template<> struct make_unsigned<char> { using type = unsigned char; };
  + template<> struct make_unsigned<short> { using type = unsigned short; };
  template<> struct make_unsigned<int> { using type = unsigned int; };
  + template<> struct make_unsigned<long> { using type = unsigned long; };
  + template<>
  + struct make_unsigned<long long> { using type = unsigned long long; };
  #endif // !FLATBUFFERS_CPP98_STL
  #else
  // MSVC 2010 doesn't support C++11 aliases.
  