Update sbt attributes, until #220 is merged

https://github.com/gildegoma/sbt-extras/blob/travis-ci/sbt is no more
compatible with sbt-extras cookbook 0.2.x, but ready for 0.3.x

[ci skip]
Update sbt-extras cookbook

Most Important Changes:
* almost ready to integrate with official paulp/sbt-extras
* sbt 0.13.0 is now default launcher
* better pre-installation of sbt and scala boot libraries
* verbose mode enabled to better show which sbt and scala versions are used during the build
* custom JVM options simplified, since sbt-extras now include some of our tweaks.