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+
Merge pull request #718 from sonatype/NEXUS-6836-bump-client-xstream

NEXUS-6836: Bump nexus-client-core XStream version to latest