Upgrade Flyway

[pierre]

The dry-run feature is a Pro feature for Flyway:

• Output to sql file instead of commiting against the DB flyway/flyway#87
• https://flywaydb.org/documentation/dryruns

Because of this, we've implemented our own dry-run on top of Flyway 4.0. Work is needed to port the code to the latest version.

Note: it looks like some folks are relying on this outside of Kill Bill (https://stackoverflow.com/a/38086304).