title: Can't install doctrine/mongodb-odm using Composer
body: <p>Trying to install doctrine/mongodb-odm via Composer I get this:</p>
  
  <pre><code>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
      - doctrine/mongodb 1.0.x-dev requires ext-mongo &gt;=1.2.12,&lt;1.4-dev -&gt; the requested PHP extension mongo is missing from your system.
      - doctrine/mongodb 1.0.2 requires ext-mongo &gt;=1.2.12,&lt;1.4-dev -&gt; the requested PHP extension mongo is missing from your system.
      - doctrine/mongodb 1.0.1 requires ext-mongo &gt;=1.2.12,&lt;1.4-dev -&gt; the requested PHP extension mongo is missing from your system.
      - doctrine/mongodb 1.0.0-BETA3 requires ext-mongo &gt;=1.3.1,&lt;1.4-dev -&gt; the requested PHP extension mongo is missing from your system.
      - doctrine/mongodb 1.0.0-BETA2 requires ext-mongo &gt;=1.2.12,&lt;1.3-dev -&gt; the requested PHP extension mongo is missing from your system.
      - doctrine/mongodb 1.0.0-BETA1 requires ext-mongo * -&gt; the requested PHP extension mongo is missing from your system.
      - doctrine/mongodb 1.0.0 requires ext-mongo &gt;=1.2.12,&lt;1.4-dev -&gt; the requested PHP extension mongo is missing from your system.
      - Installation request for doctrine/mongodb 1.0.*@dev -&gt; satisfiable by doctrine/mongodb 1.0.0, doctrine/mongodb 1.0.0-BETA1, doctrine/mongodb 1.0.0-BETA2, doctrine/mongodb 1.0.0-BETA3, doctrine/mongodb 1.0.1, doctrine/mongodb 1.0.2, doctrine/mongodb 1.0.x-dev.
  </code></pre>
  
  <p>However <code>php -m</code> has:</p>
  
  <pre><code>[PHP Modules]
  ...
  mongo
  ...
  </code></pre>
  
  <p>And <code>composer.phar show --platform</code> has:</p>
  
  <pre><code>...
  ext-mongo        1.3.5     The mongo PHP extension
  ...
  </code></pre>
  
  <p>Any ideas? The extension is definitely installed correctly, and Composer even lists it. The version (>=1.2.12,&lt;1.4-dev) meets the requirements as well.</p>