Upgrade to Scala 2.11.0-M6.

That entails tweaking dependencies once again. Hold your breath:

  * we can't upgrade to new version of partest because it's not been
    released yet; we can't release it first because scalacheck is needed
  * therefore we are using the M5 version and then we have to exclude
    all transitive dependencies that were built against M5; one such
    dependency would be scala-compiler itself
  * since we excluded all transitive dependencies we need to bring some
    back: we add diffutils and scala-compiler but M6 version

All newly added dependencies are in "test" scope. I verified that nothing
leaks to a pom.xml.

Also, updated check file for one of the tests because there were some
improvements to error handling that changed errors being emitted. See
SI-7895 for details.