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 #716 from sonatype/nexus-2.10.x-NEXUS-6836-bump-client-xstream

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