Update apt.postgresql.org key url 

Accordingly to http://wiki.postgresql.org/wiki/Apt the correc key_source url is https://www.postgresql.org/media/keys/ACCC4CF8.asc

Today the old file was removed, so it's urgent to update this file and made a new release
Merge pull request #409 from mnencia/patch-1

Update apt.postgresql.org key url