chore(builder): update heroku-buildpack-clojure to v68

See heroku/heroku-buildpack-clojure@v66...v68

Author: mboersma

builder/rootfs/usr/local/src/slugbuilder/builder/install-buildpacks

@@ -37,6 +37,6 @@ download_buildpack https://github.com/heroku/heroku-buildpack-grails.git
 download_buildpack https://github.com/heroku/heroku-buildpack-play.git           v23
 download_buildpack https://github.com/heroku/heroku-buildpack-python.git         v58
 download_buildpack https://github.com/heroku/heroku-buildpack-php.git            v67
-download_buildpack https://github.com/heroku/heroku-buildpack-clojure.git        v66
+download_buildpack https://github.com/heroku/heroku-buildpack-clojure.git        v68
 download_buildpack https://github.com/heroku/heroku-buildpack-scala.git          v60
 download_buildpack https://github.com/heroku/heroku-buildpack-go.git             6eeb09f