Upgrade to sbt 0.13.1
Switch to sbt 0.13 syntax for setting scalacOptions
Support Scala 2.11 when setting scalacOptions
Upgrade to sbt-pgp 0.8.1 release.

The 0.8 had broken dependencies (on artifacts compiled against Scala 2.9)
that caused our dbuild process to fail. The 0.8.1 release is the latest
stable release and has fixed dependencies.
Merge pull request #73 from gkossakowski/scala-2.11

Scala 2.11 compatibility