back to OpenJDK 8 due to Heroku slug size fix

Modifying the Heroku Scala Buildpack to remove `/.sbt_home/boot` brings vanilla OpenJDK 8 slug size from down from >300 MB (where application will not run) to ~200MB. Not ideal, but significantly better. Will continue to look to ways to get that down even further, short of falling back to Java 7, which has slugs of size ~140MB (this is a reasonable goal).