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
Take advantage of SBT 0.13.0
Merge pull request #61 from retronym/topic/upgrade

Ecosystem updates