slugbuilder: Update buildpacks

Updates to use the latest `jvm-common`, Clojure lein 2.50, Python 2.7.9 and Go 1.4:

* https://github.com/heroku/heroku-buildpack-clojure/compare/eb4ff18a...v63
* https://github.com/heroku/heroku-buildpack-python/compare/a51c1624...v53
* https://github.com/heroku/heroku-buildpack-scala/compare/6131f351...26cb564a
* https://github.com/heroku/heroku-buildpack-play/compare/457f4ad7...9c137b4a
* https://github.com/heroku/heroku-buildpack-php/compare/v47...v49
* https://github.com/kr/heroku-buildpack-go/compare/6ddc65c2...3a2cc0fa

Closes #604.