Allow custom BUILD_COMMAND.
Allow custom BUILD_COMMAND.

heroku/heroku-buildpack-clojure,cedricpineau/heroku-buildpack-clojure-phantomjs,heroku/heroku-buildpack-clojure,mstine/heroku-buildpack-clojure,nvbn/heroku-buildpack-clojure,jkutner/heroku-buildpack-lein,cloudControl/buildpack-clojure