REVIEW [2.10] NEXUS-6836: Bump nexus-client-core XStream version to latest

As current xstream version has a bug preventing it to work
on Oracle Java8 (XSTR-746) that is fixed in 1.4.6+

Note: nexus-client-core does NOT use the patched xstream with whitelist support.

Issue
https://issues.sonatype.org/browse/NEXUS-6836

CI
http://bamboo.s/browse/NX-OSSF252