slugbuilder: Update buildpacks

Minor bumps, nothing major.

https://github.com/heroku/heroku-buildpack-python/compare/v53...v54
https://github.com/heroku/heroku-buildpack-java/compare/83549077...e99a5f80
https://github.com/heroku/heroku-buildpack-scala/compare/26cb564a...f5415da1
https://github.com/heroku/heroku-buildpack-php/compare/v50...v51

Signed-off-by: Jonathan Rudenberg <jonathan@titanous.com>
Merge pull request #662 from flynn/update-buildpacks

slugbuilder: Update buildpacks