Switch :use to :require :refer in clojure tests

@ryanbraganza pointed out that `use` is deprecated/discouraged [1]
in favor of `require` plus `refer`.

This tripped me up, so I've updated all the clojure assignment tests
(gulp) to use the preferred form.

[1] http://dev.clojure.org/jira/browse/CLJ-879
Merge pull request #559 from jgwhite/clojure-use-to-require-refer

Switch :use to :require :refer in clojure tests