add db/schema.sql

Not super necessary, but there's a [bug in pliny](https://github.com/interagent/pliny/pull/73) causing issues to run `bin/setup` on apps without a schema. This would also make that setup a bit faster.
I gave something like this a try earlier for Travis https://github.com/heroku/telex/commit/cafe2e20076d094b6c7ee30931aca176adbbc942 

I think it was this part that made things not work on travis (and probably heroku), since you're not the owner of the plpgsql? Maybe the thing that pliny does to figure out the schema is missing --no-owner --no-acl?

For local stuff it'd be fine so I'm +1 on this, ship it.