Modify project.clj version using sjacket

Refers to: https://github.com/technomancy/leiningen/issues/1388

First attempt was a shorter implementation using `clojure.walk`, which should work just as well, but my hunch is zipping will generalise better for later modification. If that intuition is junk, let me know and I'll share that solution too.

@arr-ee, sorry for jumping in - getting to learn and get my hands dirty with sjacket, clojure.walk and clojure.zip was just too enticing.. :tada: