test alembic upgrade/downgrade in gitlab CI against postgres 10

Using a clean postgres 10 DB, this new build tests alembic upgrade head, alembic downgrade base (which should leave an empty database), then alembic upgrade head. This sequence should ensure that both the upgrade path applies cleanly, and that the downgrade path removes everything. (If anyone knows of a good way to verify that a particular postgres DB is empty, that would be a nice addition, too)

Closes #59 (closed)