title: symfony 4.0 failed to install assetic-bundle
body: <p>I'm trying this new symfony 4.0, so I failed to install assetic-bundle,
  the command I use</p>
  
  <pre><code>composer require assetic-bundle
  </code></pre>
  
  <p>I got a set of problems :</p>
  
  <pre><code>Updating dependencies (including require-dev)
  Your requirements could not be resolved to an installable set of packages.
  Problem 1
  - symfony/assetic-bundle v2.8.2 requires symfony/framework-bundle ~2.3|~3.0
  -&gt; satisfiable by symfony/framework-bundle[2.3.x-dev, 2.4.x-dev,/....../  v3.4.0-RC2]
  but these conflict with your requirements or minimum-stability.
  </code></pre>
  
  <p>I got four problems like this (I just took the important part of the first because they are very long)
  my requirements in the json file:</p>
  
  <pre><code>"require": {
      "php": "^7.1.3",
      "sensio/framework-extra-bundle": "^5.1",
      "sensiolabs/security-checker": "^4.1",
      "symfony/asset": "^4.0",
      "symfony/console": "^4.0",
      "symfony/flex": "^1.0",
      "symfony/framework-bundle": "^4.0",
      "symfony/lts": "^4@dev",
      "symfony/profiler-pack": "^1.0",
      "symfony/twig-bundle": "^4.0",
      "symfony/web-server-bundle": "^4.0",
      "symfony/yaml": "^4.0"
  },
  "require-dev": {
      "symfony/dotenv": "^4.0"
  },
  </code></pre>
  
  <p>hearing from you, thanks ^_^</p>