title: composer is ignoring installer-paths configuration
body: <p>I'm try using CakePHP for the first time with composer, but I have some problems.</p>
  
  <p>I have this <code>composer.json</code>:</p>
  
  <pre><code>{
    "name": "example.com.br",
    "repositories": [
      {
        "type": "pear",
        "url": "http://pear.cakephp.org"
      }
    ],
    "config": {
      "vendor-dir": "Vendor/"
    },
    "require": {
      "php": "&gt;=5.4",
      "pear-cakephp/cakephp": "&gt;=2.4.3",
      "cakephp/debug_kit": "2.2.*",
      "smottt/wideimage": "dev-master"
    },
    "extra": {
      "installer-paths": {
        "app/Plugin/DebugKit": ["cakephp/debug_kit"],
        "app/Vendor/Wideimage": ["smottt/wideimage"]
      }
    }  
  }
  </code></pre>
  
  <p>When I run <code>composer install (or update) --prefer-dist</code>, everything works except <code>smottt/wideimage</code>.</p>
  
  <p>This package is being installed in the <code>/Vendor</code> folder instead <code>/app/Vendor</code>, so, <strong>installer-paths</strong> was ignored.</p>