Fix lammps version number explanation

The 3rd number in the version number is not actually the day, but instead it represents the revision iteration of the month (understandably peculiar). This change will make it easier for maintainers to translate from the tar-ball name "lammps-12Jan13.tgz" to a version number "2013.01.12", which was released on January 11.

Because I was already making changes, I updated the tar-ball to the latest version.