Simplify finding the distribution to release.

The scala-dist-{unix, windows} jobs are run downstream from scala-dist.
The latter builds a distribution once, which is then packaged for the
two platforms. The distribution is put in the target/dist/tmp by the
Jenkins job so we don't have pull crazy tricks in the SBT build.
Merge pull request #83 from adriaanm/release-simplify

Simplify finding the distribution to release.