Impossible to use wifiClient

Have you upgraded the firmware? Check with this code after Serial.begin() in your setup. If upgraded, it should show version 1.1.0.

  // check for the presence of the shield:
  if (WiFi.status() == WL_NO_SHIELD) {
    Serial.println("WiFi shield not present"); 
    // don't continue:
    while(true);
  } 

  // check firmware version
  Serial.print(F("Firmware version: "));
  Serial.println(WiFi.firmwareVersion());