Ecosystem updates:

- move to SBT 0.13, and corresponding versions of sbt-mima and
   sbt-pgp
 - move from Scala 2.10.2 to 2.10.3
 - update the milestone version of of 2.11
 - drop dependency on scala-actors in favour of scala.concurrent,
   which is available in Scala 2.9.3 and higher
   - degrade gracefully on 2.9.2 by running sequentially.

The SBT update means we get correct handling of scalaBinaryVersion for
non-final releases:

    [info] Set current project to scalacheck (in build
file:/Users/jason/code/scalacheck/)
   > ++2.11.0-M5
   [info] Setting version to 2.11.0-M5
   [info] Set current project to scalacheck (in build
file:/Users/jason/code/scalacheck/)
   > scalaBinaryVersion
   [info] 2.11.0-M5

This prevents errors like:

    sbt> ++ 2.11.0-M5
   sbt> update
   [error] (*:update) sbt.ResolveException: unresolved dependency:
org.scala-lang.modules#scala-parser-combinators_2.11;1.0-RC2: not found