Updated composer dependencies. Now using Symfony2.6

Hi friend. Really appreciated your project to dive into Symfony2. 

This pull request consists of 3 small commits,

1. I deleted the Upgrade-*.md files, they are for the older 2.2 and 2.3 versions of Symfony.

2. I updated composer.json to use Symfony 2.6, and from the 2.6 composer file, updated the helper libraries while I was at it.

3. Ran composer.json, commit the lock file and the 2 files that were changed in /app by this operation:
      app/SymfonyRequirements.php
      app/check.php

Thanks