chore(builder): update heroku-buildpack-java to v39

See heroku/heroku-buildpack-java@v38...v39

Author: mboersma

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

@@ -31,7 +31,7 @@ mkdir -p $BUILDPACK_INSTALL_PATH
 download_buildpack https://github.com/heroku/heroku-buildpack-multi.git          26fa21a
 download_buildpack https://github.com/heroku/heroku-buildpack-ruby.git           v137
 download_buildpack https://github.com/heroku/heroku-buildpack-nodejs.git         v75
-download_buildpack https://github.com/heroku/heroku-buildpack-java.git           v38
+download_buildpack https://github.com/heroku/heroku-buildpack-java.git           v39
 download_buildpack https://github.com/heroku/heroku-buildpack-gradle.git         v12
 download_buildpack https://github.com/heroku/heroku-buildpack-grails.git         v19
 download_buildpack https://github.com/heroku/heroku-buildpack-play.git           v24