establish a database migration strategy

[adamdecaf]

Right now keeping a postgres database for tls-observatory is an artisanal chore. We should automate it instead so as to not create issues for users deploying their own instances.
(See #317)

Personally I've used https://github.com/rubenv/sql-migrate before, but I'm sure there are a wealth of other options. awesome-go offers a few other migration tools.

[jvehent]

I'm ok with using a migration tool, especially if its automated (deploying the production service is fully automated so it'd be nice to not require manual commands).
If you already have experience with a specific tool, would you mind proposing a patch?