restrict domainResource usage

Author: jenschude

commercetools/commercetools-sdk-java-api/src/main/java/com/commercetools/api/models/DomainResource.java

@@ -1,6 +1,6 @@
 package com.commercetools.api.models;
 
-public interface DomainResource<T> {
+public interface DomainResource<T extends DomainResource<T>> {
     public String getId();
 
     public Long getVersion();