title: "composer install" invokes post-update-cmd instead of post-install-cmd
body: <p>I have this simple composer.json file. When I run "composer install" I expect it to run post-install-cmd script but nothing happens. If I change it to post-update-cmd it works as expected. Am i missing something? Doesn't it have to run post-install-cmd instead of post-update-cmd?</p>
  
  <pre><code>{
      "name": "vendorName/packageName",
      "description": "Some description",
      "type": "library",
      "require": {
          "psr/log": "^1.1"
      },
      "scripts": {
          "post-install-cmd": "$SHELL script.sh"
      }
  }
  </code></pre>