database.yml

core: &core
  adapter:  postgis
  encoding: unicode
  host:     127.0.0.1
  username: postgres
  database: magnum<%= ENV['CI_BUILD_NUMBER'] %>

development:
  <<: *core
test:
  <<: *core
staging:
  <<: *core
production:
  <<: *core

install our own postgres and postgis

sudo sh -c 'echo "deb http://apt.postgresql.org/pub/repos/apt/ $(lsb_release -cs)-pgdg main" > /etc/apt/sources.list.d/pgdg.list'
sudo apt-get update -y
sudo apt-get install postgresql-9.3-postgis-2.1 postgresql-9.3 postgresql-server-dev-9.3 -y
sudo ldconfig
sudo service postgresql start

create and extend the db

psql -c "create database magnum$CI_BUILD_NUMBER;" -U postgres
psql -c "CREATE EXTENSION postgis; CREATE EXTENSION postgis_topology;" magnum$CI_BUILD_NUMBER

grab this database.yml

curl -o config/database.yml 'https://gist.githubusercontent.com/cmhobbs/a25b31b18dade3d54443/raw/fd0411c8dbd36e7e434af974b29ec3e1a5a5ac58/database.yml'

Note that magnum-ci will always try to run bundle exec rake db:create and it will always fail. I'm trying to find ways to get rid of that step in their configuration.