Update postgres_default_version to 9.1 for debian 7.0

/etc/debian_version on Wheezy was updated to 7.0 with the release of the
base-files package on 2012-12-12, which means that wheezy could be
either 7.0 or wheezy depending on what version of base-files is
installed. To handle both cases we treat 'wheezy' and '7.*' as
synonymous.