Remove heroku/heroku-buildpack-php from dev dependencies (#88)

* remove heroku/heroku-buildpack-php from dev dependencies

not used for anything

* another composer update

Author: dzuelke

composer.json

@@ -8,8 +8,5 @@
     "monolog/monolog": "^3.0",
     "twig/twig": "^3.0",
     "slim/twig-view": "^3.0"
-  },
-  "require-dev": {
-    "heroku/heroku-buildpack-php": "*"
   }
 }