Add missing backslashes

Note that \lib includes backslash, and it's more or less standard on Windows to not include backslash in environment variable paths. Otherwise get errors like so -

> script\repl
Clojure 1.5.1
user=> (require '[cljs.repl :as repl])
FileNotFoundException Could not locate cljs/repl__init.class or cljs/repl.clj on classpath:   clojure.lang.RT.load (RT.j
ava:443)
user=>