fix UT failure in com.alibaba.dubbo.registry.multicast.MulticastRegistryTest on JDK 7 and above

pom.xml

@@ -120,7 +120,7 @@
 		<easymock_version>3.0</easymock_version>
 		<jmockit_version>0.999.8</jmockit_version>
 		<!-- Build args -->
-		<argline>-server -Xms64m -Xmx256m -XX:PermSize=64m -XX:MaxPermSize=128m -Dfile.encoding=UTF-8</argline>
+		<argline>-server -Xms64m -Xmx256m -XX:PermSize=64m -XX:MaxPermSize=128m -Dfile.encoding=UTF-8 -Djava.net.preferIPv4Stack=true</argline>
 		<skip_maven_deploy>false</skip_maven_deploy>
 		<updateReleaseInfo>true</updateReleaseInfo>
 		<project.build.sourceEncoding>${file_encoding}</project.build.sourceEncoding>