title: Heroku is not setting php version from my composer file
body: <p>I did follow the guide to deploy a php project in <code>heroku</code> i am trying to deploy a <code>laravel 5.6</code> project, i did everything it says to do and my <code>composer</code> looks like this</p>
  
  <pre><code>{
      "name": "My project",
      "description": "my project description.",
      "keywords": ["framework", "laravel"],
      "license": "MIT",
      "type": "project",
      "require": {
          "php": "^7.1.0",
          "fideloper/proxy": "^4.0",
          "intervention/image": "^2.4",
          "laravel/framework": "5.6.*",
          "laravel/tinker": "^1.0",
          "maatwebsite/excel": "~2.1.0"
      },
  }
  </code></pre>
  
  <p>as you can see i am setting <code>php ^7.1.0</code> but when i make push to <code>heroku</code> i see this</p>
  
  <pre><code> Installing platform packages...
   remote:        - php (7.2.9)
   remote:        - ext-mbstring (bundled with php)
   remote:        - nginx (1.8.1)
   remote:        - apache (2.4.34)
  </code></pre>
  
  <p>what i want is to use php 7.1. What am i missing? do i have to add the php in another require?</p>