proguard.cfg update for Scala 2.10.2

In Scala 2.10.2 Fork/Join pool changed, new proguard.cfg mirrors
those changes for members accessed trough introspection.

See https://issues.scala-lang.org/browse/SI-7442 for details.
Merge pull request #22 from aginiewicz/proguard-2.10.2

proguard.cfg update for Scala 2.10.2