title: Composer dependency conflict
body: <p>Running this <code>composer require intervention/image</code> throws this error:</p>
  
  <pre><code>Using version ^2.3 for intervention/image
  ./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
  - Conclusion: don't install intervention/image 2.3.1
  - Conclusion: don't install intervention/image 2.3.2
  - Installation request for intervention/image ^2.3 -&gt; satisfiable by intervention/image[2.3.0, 2.3.1, 2.3.2].
  - intervention/image 2.3.0 requires guzzlehttp/psr7 ~1.1 -&gt; satisfiable by guzzlehttp/psr7[1.1.0, 1.2.0].
  - guzzlehttp/psr7 1.1.0 requires psr/http-message ~1.0 -&gt; satisfiable by psr/http-message[1.0].
  - guzzlehttp/psr7 1.2.0 requires psr/http-message ~1.0 -&gt; satisfiable by psr/http-message[1.0].
  - Conclusion: don't install psr/http-message 1.0
  
  
  Installation failed, reverting ./composer.json to its original content.
  </code></pre>
  
  <p>So it's caused by Guzzle (I'm using <code>"guzzlehttp/guzzle": "~5.3"</code>). But I can't (or don't want) to update guzzle to 6.0 because it's not backward compatible, so I have to move a lot of code. Can I do something, like a trick, or do I have to update <code>guzzlehttp/guzzle</code> or <code>intervention/image</code>?</p>