Bump to Leiningen 2.3.4.
Bump to Leiningen 2.3.4.

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