Use the Postgres gem and remove usernames and passwords from db.yml

- This copies Transition's behaviour on the postgres-master branch.
Change the Jenkins job for CI to use Postgres

- Set up the database with Postgres.
- Dump `db/structure.sql`.
Downcase host for PostgreSQL

PostgreSQL is case-sensitive for queries: 'www.MINITRUE.gov.uk' is
not the same as 'www.minitrue.gov.uk'. This change fixes all 5 failing
tests after switching to PostgreSQL.
Merge pull request #98 from alphagov/postgres_migration

Postgres migration