Detect modified migration scripts

[felixkrull-neuland]

When comparing the local migration scripts with the record of already the executed scripts, the library should be able to detect if an already-executed script was modified afterwards, and raise an error. Given that checksums of the scripts are calculated and stored, I was surprised that this wasn't already being checked. I'm not that familiar with Flyway, but the Play Framework's evolutions module does verify the checksum.

Is this an intentional design choice? If not, I'd change it and open a PR.