Make CI builds more reliable

This fixes the following scenario:
 1) schema change made in Transition app to the shared database
 2) bouncer depends on this schema change, but is building on a box which hasn't
    run that schema change yet
 3) bouncer build fails

We already use db/structure.sql for Travis builds, let's use it for CI too.

This does highlight that db/structure.sql can become out of date. That's a
problem for another day.