Bump launcher to support multiple sbt versions.

* Launcher now uses auto-detected scala version for both the UI and sbt.
* Launcher now reads project/build.properties for sbt version before
  dropping to activator default.
Bump sbt-remote-control version to the variant that supports sbt 0.13 + atmos.

* Use sbt 0.13.0 for launcher.
* Bump sbt-remote-control version.
Quick refactoring of SBT_VERSION property to better represent that it's just a default.
Merge pull request #13 from typesafehub/wip/sbt-0.13-support

This sbt goes to 11... err... 13.