I had a look at your build.sbt (here https://github.com/dhoss/steel/blob/master/build.sbt), and there are a couple of things you should change. First, remove the dependency to jdbc module. You are using the play-slick module for all database accesses, so you should not need to Play jdbc. In fact, as soon as you update to play-slick 1.0.0-RC2 (which was released last week, together with Play 2.4.0-RC3), you will get an exception similar to https://playframework.com/documentation/2.4.0-RC3/PlaySlickFAQ#A-binding-to-play.api.db.DBApi-was-already-configured. Second, update to play-slick 1.0.0-RC2 because RC1 had an annoying issue (https://github.com/playframework/play-slick/issues/245) that could prevent starting your app.
Otherwise, it's a good article. I'll see how I can clarify why you need both
in your application.conf. But, in a nutshell, the first is the Slick driver (which you will be using in your code), while the second is the JDBC driver which is going to be used by Slick backend. See the Slick documentation for DatabaseConfig http://slick.typesafe.com/doc/3.0.0/database.html#databaseconfig
Thanks for trying it out, and taking the time to write about your experience.