new v8 deprecation cause build failing #167

../src/node_file.cc:1940:59: error: 'IsExternal' is deprecated: Use String::IsExternalTwoByte() or String::IsExternalOneByte() [-Werror,-Wdeprecated-declarations]
    } else if (enc == UCS2 && IsLittleEndian() && string->IsExternal()) {
                                                          ^
../deps/v8/include/v8.h:3121:3: note: 'IsExternal' has been explicitly marked deprecated here
  V8_DEPRECATE_SOON(
  ^
../deps/v8/include/v8config.h:405:39: note: expanded from macro 'V8_DEPRECATE_SOON'
# define V8_DEPRECATE_SOON(message) [[deprecated(message)]]