Switch to mainline of SBT 0.13

Now that https://github.com/sbt/sbt/pull/1092 is merged

Review by @cunei / @jsuereth

/cc @adriaanm, @gkossakowski, to remind us that we're still
[flying blind](https://jenkins-dbuild.typesafe.com:8499/job/Community-2.11.x/42/consoleFull)
Merge pull request #1 from retronym/topic/sbt-0.13

Switch to mainline of SBT 0.13
Remove references to now-removed akka sub projects.

    [info] [error] These subprojects were not found: "akka-channels-tests", "akka-sbt-plugin", "akka-sample-cluster", "akka-channels-experimental".
    [info] [error] Use 'last' for the full log.
Merge pull request #2 from retronym/topic/akka-subs

Remove references to now-removed akka sub projects.
More maintainence of the akka sub-project lists

    [info] [error] These subprojects were not found: "akka-sample-camel", "akka-sample-hello", "akka-sample-remote".
    [info] [error] Use 'last' for the full log.
Merge pull request #4 from retronym/topic/akka-subs2

More maintainence of the akka sub-project lists
More akka sub-project maintainence

Trying to solve:

    [error] Fatal: multiple projects produce the same artifacts. Please exclude them from some of the conflicting projects.

    [error]   com.typesafe.akka#akka-sample-main-scala, from:  akka, akka-bad
    [error]   com.typesafe.akka#akka-sample-main-java, from:  akka, akka-bad
    [error]   com.typesafe.akka#akka-sample-camel-scala, from:  akka, akka-bad
    [error]   com.typesafe.akka#akka-sample-camel-java, from:  akka, akka-bad
    [error]   com.typesafe.akka#akka-sample-remote-scala, from:  akka, akka-bad
    [error]   com.typesafe.akka#akka-sample-remote-java, from:  akka, akka-bad
    [error]   com.typesafe.akka#akka-sample-cluster-scala, from:  akka, akka-bad
    [error]   com.typesafe.akka#akka-sample-cluster-java, from:  akka, akka-bad
Merge pull request #5 from retronym/topic/akka-subs3

More akka sub-project maintainence
Akka master on Scala master