Multiple fixes: XStream on Java8, zapper...

NEXUS-6836: Bump XStream to fix it on java8
NEXUS-6728: Use updated zapper for preemptive auth
Bumped deps

Issues
https://issues.sonatype.org/browse/NEXUS-6836
https://issues.sonatype.org/browse/NEXUS-6728

CI
http://bamboo.s/browse/NX-ANTF1