Twig filesystem loader

Since Symfony 2.2 `twig.loader` service is defined dynamically by checking the count of services tagged with the same name. For example if you were to use multiple loaders then `twig.loader` becomes an alias of `twig.loader.chain` and currently causes problems with `ExtendsAdmingeneratedExtension` as it expects a `Twig_Loader_Filesystem`.

Specifying the filesystem loader explicitly resolves the issue but needs at least Symfony 2.2.

I've changed the `composer.json` accordingly.