[MASSEMBLY-712] Add symlink support by updating plexus archiver

git-svn-id: https://svn.apache.org/repos/asf/maven/plugins/trunk@1628092 13f79535-47bb-0310-9956-ffa450edef68
Updated to plexus-archiver 2.6.1, plexus-io 2.1.1 and plexus-utils 3.0.18

Full symlink support on *NIX (java7+ required), full file attribute support (java7+ required)

git-svn-id: https://svn.apache.org/repos/asf/maven/plugins/trunk@1628095 13f79535-47bb-0310-9956-ffa450edef68