title: Installation failed for phpunit/phpunit ^6.2
body: <p>when i run</p>
  
  <pre><code>composer require --dev phpunit/phpunit
  </code></pre>
  
  <p>i get the following message:</p>
  
  <pre><code>composer require --dev phpunit/phpunit
  Using version ^6.2 for phpunit/phpunit
  ./composer.json has been updated
  Loading composer repositories with package information
  Updating dependencies (including require-dev)
  Your requirements could not be resolved to an installable set of packages.
  
  Problem 1
  - Installation request for sebastian/object-enumerator (locked at 2.0.1) -&gt; satisfiable by sebastian/object-enumerator[2.0.1].
  - phpunit/phpunit 6.2.0 requires sebastian/object-enumerator ^3.0.2 -&gt; satisfiable by sebastian/object-enumerator[3.0.2].
  - phpunit/phpunit 6.2.1 requires sebastian/object-enumerator ^3.0.2 -&gt; satisfiable by sebastian/object-enumerator[3.0.2].
  - phpunit/phpunit 6.2.2 requires sebastian/object-enumerator ^3.0.2 -&gt; satisfiable by sebastian/object-enumerator[3.0.2].
  - phpunit/phpunit 6.2.3 requires sebastian/object-enumerator ^3.0.2 -&gt; satisfiable by sebastian/object-enumerator[3.0.2].
  - Conclusion: don't install sebastian/object-enumerator 3.0.2
  - Installation request for phpunit/phpunit ^6.2 -&gt; satisfiable by phpunit/phpunit[6.2.0, 6.2.1, 6.2.2, 6.2.3].
  
  Installation failed, reverting ./composer.json to its original content.
  </code></pre>
  
  <p>Here is my composer json file:</p>
  
  <pre><code>"require": {
    "php": "^7.0",
    "laravel/framework": "^5.4",
    "guzzlehttp/guzzle": "^6.3",
    "symfony/psr-http-message-bridge": "^1.0",
    "mcamara/laravel-localization": "^1.2",
    "laravelcollective/html": "^5.4",
    "laravel/socialite": "^3.0",
    "yajra/laravel-datatables-oracle": "^7.9"
  },
  "require-dev": {
    "fzaninotto/faker": "^1.6",
    "symfony/css-selector": "^3.3",
    "symfony/dom-crawler": "^3.3"
  }
  </code></pre>
  
  <p>I tried to get from version 5.4 above and always get a similar error but with other dependecies, the only version that seems to work is 5.0</p>