title: png_compression_level option should be an integer from 0 to 9
body: <p>I am using Symfony 2.3.* and I got this error in my app/logs/dev.log when I'm using LiipImagineBundle.</p>
  
  <pre><code>request.CRITICAL: Uncaught PHP Exception Imagine\Exception\InvalidArgumentException: "png_compression_level option should be an integer from 0 to 9" at /vendor/imagine/imagine/lib/Imagine/Gd/Image.php line 535 {"exception":"[object] (Imagine\\Exception\\InvalidArgumentException: png_compression_level option should be an integer from 0 to 9 at /vendor/imagine/imagine/lib/Imagine/Gd/Image.php:535)"} []
  </code></pre>
  
  <p>Any solution ? Thanks</p>
  
  <p>And here is my config</p>
  
  <pre><code>liip_imagine:
      resolvers:
         default:
            web_path: ~
      filter_sets:
          cache: ~
          standard:
              quality: 200
              filters:
                  thumbnail: { size: [400, 300], mode: outbound }
  </code></pre>