update dumbo to support repeatable migrations

[rolang]

Since v0.4.0 dumbo supports repeatable migrations, but it is not released for Scala 2.12 to be used with sbt 1. I'd try to use the dumbo command line binary instead of adding it as build dependency in sbt.

[rolang]

Another solution might be to just use it via docker command line instead: https://hub.docker.com/r/rolang/dumbo . Not very efficient, but should be easier to apply across platforms and the docker image is small (~12 MB compressed).

[rolang]

Support added in #23