Version bump sbt to 0.13.5.

This is a simple change to version bump the Dockerfile to use sbt 0.13.5. This is the https version of the link currently used on the [Installing sbt on Linux](http://www.scala-sbt.org/0.13/tutorial/Installing-sbt-on-Linux.html) page (because https is really what they should be using :unamused:).