Update the assembly plugin to the latest version.

Update the assembly plugin, used to generate the jar with dependencies, to
the latest released (non-beta) version.  No functional change.

git-svn-id: https://groke.mcmaster.ca/svn/grad/colin/branches/TableTool_javization@10720 57e6efec-57d4-0310-aeb1-a6c144bb1a8b