Upgrade project's build to SBT 0.13

	* project/AkkaBookBuild.scala (AkkaBookBuild.multiJvmSettings): In
	SBT 0.13, executeTests returns type sbt.Tests.Output and
	processResults.  Use sbt.Tests.processResults instead of
	sbt.Tests.overall.

	* project/plugins.sbt: Update SBT plugins: sbteclipse, ensime,
	multi-jvm and sbt-idea.