Use PostgreSQL for Travis CI test runs.

We've encountered a couple of production bugs due to using different databases in test and production. We're using SQLite in the test environment to ease setup for newcomers, which fits with our broader goals; this is an attempt to see if we can test on Postgres at the CI stage to catch such bugs anyway.

Code taken from http://stackoverflow.com/questions/9321153/rails-database-setup-on-travis-ci .