Fixed a cache race condition

If two people visit the page using the rss feed at the same time, there is a (small) chance of this happening:

[2014-03-13 09:26:22] request.CRITICAL: Uncaught PHP Exception Desarrolla2\Cache\Exception\FileCacheException: "/app/cache/rss-client-bundle is not writable" at /vendor/desarrolla2/cache/src/Desarrolla2/Cache/Adapter/File.php line 40

Hence a cache warmer.

http://symfony.com/doc/current/reference/dic_tags.html#kernel-cache-warmer