avoid generating an over 20k lines JS file

modify compiler's optimizations option to :advanced

ref.
  https://github.com/emezeske/lein-cljsbuild/blob/1.0.2/sample.project.clj
  http://lukevanderhart.com/2011/09/30/using-javascript-and-clojurescript.html
  https://twitter.com/mizchi/status/437276796178804736
  https://twitter.com/mizchi/status/437277484464099328
  https://twitter.com/mizchi/status/437278736681926656