chore(buildpacks): update heroku-buildpack-php to v121

See heroku/heroku-buildpack-php@v120...v121

Author: mboersma

rootfs/builder/install-buildpacks

@@ -35,7 +35,7 @@ download_buildpack https://github.com/heroku/heroku-buildpack-gradle.git
 download_buildpack https://github.com/heroku/heroku-buildpack-grails.git         v21      05-grails
 download_buildpack https://github.com/heroku/heroku-buildpack-java.git           v51      06-java
 download_buildpack https://github.com/heroku/heroku-buildpack-nodejs.git         v101     07-nodejs
-download_buildpack https://github.com/heroku/heroku-buildpack-php.git            v120     08-php
+download_buildpack https://github.com/heroku/heroku-buildpack-php.git            v121     08-php
 download_buildpack https://github.com/heroku/heroku-buildpack-play.git           v26      09-play
 download_buildpack https://github.com/heroku/heroku-buildpack-python.git         v102     10-python
 download_buildpack https://github.com/heroku/heroku-buildpack-ruby.git           v155     11-ruby