added Serializable interface, need it for Azul JDK

There is a problem when running infinispan testuite with Azul JDK, xstream library complies 
Stacktrace
com.thoughtworks.xstream.converters.ConversionException: Cannot construct org.infinispan.container.entries.ImmortalCacheValue as it does not have a no-args constructor : Cannot construct org.infinispan.container.entries.ImmortalCacheValue as it does not have a no-args constructor
---- Debugging information ----
message             : Cannot construct org.infinispan.container.entries.ImmortalCacheValue as it does not have a no-args constructor
cause-exception     : com.thoughtworks.xstream.converters.reflection.ObjectAccessException
cause-message       : Cannot construct org.infinispan.container.entries.ImmortalCacheValue as it does not have a no-args constructor
class               : org.infinispan.container.entries.ImmortalCacheValue
required-type       : org.infinispan.container.entries.ImmortalCacheValue
converter-type      : com.thoughtworks.xstream.converters.reflection.ReflectionConverter
path                : /org.infinispan.container.entries.ImmortalCacheValue
line number         : 1
version             : 1.4.1-redhat-2
You can see it here https://jenkins.mw.lab.eng.bos.redhat.com/hudson/view/JDG/view/FUNC/job/jdg-62-ispn-testsuite-rhel-azul/4/#showFailuresLink