Use scala-partest 1.0-RC4

RC3 had a binary incompatibility that led to a crash in parsing options,
due to:
- https://github.com/scala/scala-partest/commit/9ff138ab4b
- https://github.com/scala/scala/commit/2b1563fa74

Also added sonatype as a repo for resolving partest,
so we don't have to wait for artifacts to synch to maven central.