Use the same version of ActiveRecord as transition

- This forces a `mysql2` gem upgrade too, as running the tests doesn't
  work with 0.3.11. The `mysql2` gem is also now the same as transition.
Merge pull request #88 from alphagov/use_same_activerecord_version_as_transition

Use the same version of ActiveRecord as transition
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`.