Bump Scala version to latest master.
Add work-around for duplicated source files in Play build.

For reason I don't really understand, PlayVersion.scala is being passed
twice to scalac when compiling Play2-core with dbuild. Let's work around
it for now until I can understand what's going on.
Update to sbt 0.13.1 in scala-io

My fork switched to sbt 0.13.1. Let's mention this.
Work-arounds for compiling scala-io.

Add work-around for scala compiler crash described in SI-8306. Also,
do not run tests because one of them fails for some reason.