slugbuilder: Update buildpacks

https://github.com/ddollar/heroku-buildpack-multi/compare/93505719...6e790943
https://github.com/heroku/heroku-buildpack-ruby/compare/v127...v129
https://github.com/heroku/heroku-buildpack-python/compare/1525c1fb...a51c162
https://github.com/heroku/heroku-buildpack-java/compare/0f916f93...83549077
https://github.com/heroku/heroku-buildpack-gradle/compare/f9127a26...24a8ebe7
https://github.com/heroku/heroku-buildpack-scala/compare/199db6eb...6131f351
https://github.com/heroku/heroku-buildpack-php/compare/v46...v47
https://github.com/kr/heroku-buildpack-go/compare/b261aab1...6ddc65c2
website: Update default JDK version to 8

Signed-off-by: Jonathan Rudenberg <jonathan@titanous.com>