Need an "import" in 04-checker.md

[A. Jesse Jiryu Davis]

I'm using Clojure for the first time in order to work through the tutorial. Following instructions in the "Checking Correctness" section, I got:

Syntax error compiling deftype* at (jepsen/foo.clj:92:1).
Unable to resolve symbol: Model in this context

I think we need (:import (knossos.model Model)) or something like that.

[Kyle Kingsbury]

That seems reasonable! Want to shoot over a PR?