It would be good if the Warp can be compilable with older JDK.

The project is not compilable with jdk 6 because of method which was added in java 7 used on this line.
Imho it could be replaced with:

try {
  return Inet4Address.getByName(null);
} catch (UnknownHostException ex) {
  throw new RuntimeException(ex);
}

From the javadoc of this method:

If the host is null then an InetAddress representing an address of the loopback interface is returned. See RFC 3330 section 2 and RFC 2373 section 2.5.3.