Allow platform packages in composer.json

From now on, Debby didn't really understand platform packages: packages like "php": ">=5.4.0" or "ext-curl": "*". It would check these packages at packagist and would fail. Now, these are skipped without furthers checks. They are not updatable in the same sense as normal packages.

Maybe later Debby actually checks them for outdated installs as well, but that might be a bit more tricky :)