Bump to latest pgp plugin.
Minor build cleanups.

1. We only need to cross publish the latest version for each Scala BC series.
   Otherwise, you get duplicate artifacts pushed to maven central and we're
   not sure which ones win.
2. no longer need to use seq() in settings in sbt 0.12.
Merge pull request #5 from jsuereth/wip/scala-2.9.x-support

Wip/scala 2.9.x support