GROOVY-9898: Bump xstream to 1.4.15
GROOVY-9898: Bump xstream to 1.4.15

paulk-asert/groovy,apache/groovy,apache/groovy,apache/incubator-groovy,apache/groovy,paulk-asert/groovy,paulk-asert/groovy,paulk-asert/groovy,apache/groovy,apache/incubator-groovy,apache/incubator-groovy,apache/incubator-groovy