chore(buildpacks): update heroku-buildpack-php to v108

See heroku/heroku-buildpack-php@v107...v108

Author: mboersma

rootfs/builder/install-buildpacks

@@ -36,7 +36,7 @@ download_buildpack https://github.com/heroku/heroku-buildpack-gradle.git
 download_buildpack https://github.com/heroku/heroku-buildpack-grails.git         v20
 download_buildpack https://github.com/heroku/heroku-buildpack-play.git           v26
 download_buildpack https://github.com/heroku/heroku-buildpack-python.git         v80
-download_buildpack https://github.com/heroku/heroku-buildpack-php.git            v107
+download_buildpack https://github.com/heroku/heroku-buildpack-php.git            v108
 download_buildpack https://github.com/heroku/heroku-buildpack-clojure.git        v75
 download_buildpack https://github.com/heroku/heroku-buildpack-scala.git          v70
 download_buildpack https://github.com/heroku/heroku-buildpack-go.git             v42