Set the Heroku BUILDPACK_URL var, thanks to @Salakar

app.json

@@ -14,7 +14,8 @@
 	"keywords": ["cachet", "laravel", "status", "page"],
 	"addons": ["cleardb"],
 	"env": {
-		"ENV": "heroku"
+		"ENV": "heroku",
+		"BUILDPACK_URL": "https://github.com/heroku/heroku-buildpack-php"
 	},
 	"scripts": {
 		"postdeploy": "php artisan migrate; php artisan key:generate"

package.json

@@ -0,0 +1,6 @@
+{
+    "devDependencies": {
+        "gulp": "^3.8.8",
+        "laravel-elixir": "*"
+    }
+}