Fixes support for scala-pr-validator

Update the script to support the Scala PR validator process.
Based on #21
@adriaanm I think it covers the changes made in #19. I didn't add the `extraDbuildArgs` in the zinc section, because it works fine without it in my local tests.
If you could give it a try in the Scala jenkins, that would be great.
I put some documentation on how to run the script in the README. I tried to emulate your process.