title: Installing or updating symfony/flex via Composer silently fails
body: <p>I have a project using symfony/flex in which I cannot run <code>composer install</code> or <code>composer update</code>. I also can't create a new composer project with symfony/flex as a dependency as the failure is the same. For example;</p>
  
  <pre><code>mkdir foo &amp;&amp; cd foo &amp;&amp; composer require symfony/flex -vvv
  </code></pre>
  
  <p>Always has the following output;</p>
  
  <pre><code>...
  Package operations: 1 install, 0 updates, 0 removals
  Installs: symfony/flex:v1.2.6
    - Installing symfony/flex (v1.2.6): Reading /Users/me/.composer/cache/files/symfony/flex/17b622276922e6347ea129acd6238ae1c41d96b6.zip from cache
  Loading from cache
   Extracting archiveExecuting command (CWD): unzip -qq  '/Users/me/projects/foo/vendor/symfony/flex/2064a553f7cdf064ba8c1b62c7087ec6' -d '/Users/me/projects/foo/vendor/composer/65215880'
  Loading plugin Symfony\Flex\Flex
  Downloading https://flex.symfony.com/versions.json
  </code></pre>
  
  <p>The download hangs for a few seconds at the point of downloading versions.json before silently failing. <code>vendor/symfony/flex</code> exists.</p>
  
  <p>What could be the cause of this failure? My colleague can run the same command successfully so I'm assuming an issue with my local composer. I have attempted reinstall composer and clearing the cache with no success.</p>
  
  <p><strong>Edit:</strong> now when running <code>composer update -vvv</code> in an existing project I get the following output</p>
  
  <pre><code>...
  Downloading https://repo.packagist.org/packages.json
  </code></pre>
  
  <p>Where it also hangs, so it looks like composer cannot download certain JSON files? Packages download fine so it doesn't look like a lack of internet.</p>
  
  <p><strong>Edit 2:</strong> I can also confirm using <code>composer update --no-plugins</code> does work within the existing project.</p>