don't need actors
Honor Java prop `prRepoUrl` for maven resolution.

PR validation uses an internal artifactory for build artifacts.
This provides a hook to make dbuild look there.
The default is a duplicate of another repo, so that we don't fail
when prRepoUrl isn't set. I couldn't find a more elegant way.
Merge pull request #14 from adriaanm/master

Give PR validation a hook to tell dbuild where to resolve scala from