EPOCH=1 needs to be set for Ubuntu 18.10 cosmic builds

MariaDB 10.1 builds for cosmic in buildbot are failing install tests at least in part because Ubuntu's MariaDB 10.1 since Ubuntu Bionic has EPOCH set to 1.

We added code for Ubuntu Bionic to set EPOCH=1 in debian/autobake-deb.sh and that section needs to be extended to also include Ubuntu Cosmic.

Instead of just adding cosmic to a static list, a better fix would be to set the EPOCH for any Ubuntu build greater than 18.04, that way we wouldn't have to do this again. I have a patch that does this, but I need to test it first.