By future-proof I didn't actually mean forever. It heavily makes use of es6/es7, so it was designed with "forward-thinking" design choices (forward thinking is what I intended when I wrote future proof). Right now it relies on babel, but as es6/es7 gains adoption it'll grow out of requiring it. Similar to Angular 2.0, which is also being developed with a forward thinking mindset.
I used Scalatra on one other project, and I found it to be difficult to deploy. Whether it's true or not, my impression is that Spark is a little simpler to use and deploy than Scalatra. Scalatra is my second choice though.