Update versions in changelog

[scpeters]

There are some new debs built for version 6.8.0 but they are labeled as 6.7.2; I assume because the latest entry in the changelog files is 6.7.2.

• https://github.com/dartsim/dart-debian/blob/master/bionic/debian/changelog
• https://launchpad.net/~dartsim/+archive/ubuntu/ppa/+packages

We use the following script for updating changelog entries for our software, so you may find it useful:

• https://bitbucket.org/osrf/release-tools/src/b6ae2f2928c45458f30d365a632a870fa8050034/release-repo-scripts/changelog_spawn.sh?at=default&fileviewer=file-view-default

[jslee02]

The script is super useful, thanks! I modified it a bit for git and pushed it.