One concern of mine here is that the JS API for `node-db-migrate` isn't actually publicly documented; only [its command-line](https://github.com/nearinfinity/node-db-migrate#usage) is. The implementation for `exports.migrations.up`/`down` here was actually cribbed from the [bin/db-migrate](https://github.com/nearinfinity/node-db-migrate/blob/53ef390074f3541ce5d4ca37417feedb8d69defc/bin/db-migrate#L103) source code, so I'm not sure if/when it'll break, but at least the migration test I added exercises this code reasonably well.