chore(builder): update heroku-buildpack-multi to 26fa21a

Author: mboersma

builder/image/slugbuilder/builder/install-buildpacks

@@ -28,7 +28,7 @@ download_buildpack() {
 
 mkdir -p $BUILDPACK_INSTALL_PATH
 
-download_buildpack https://github.com/ddollar/heroku-buildpack-multi.git         6e79094
+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