repo.sovrin.org should be a higher priority repo

Besides telling stewards to add repo.sovrin.org to their /etc/apt/sources.lst, we also tell them to configure repo.sovrin.org to be a higher priority repo (from apt's perspective) than the default ubuntu repos, using instructions from askubuntu. (We should automate this by putting it in sovrin.deb's postinst script). (I've logged a ticket to develop an equivalent solution for RHEL.)