Check exclude from downloaded composer.json

Hi !

I met a problem when archiving a repo from packagist.

https://github.com/liip/LiipImagineBundle has a [composer archive property](http://getcomposer.org/doc/04-schema.md#archive) which excludes a directory from the archive. But it seems it does not work because packagist doesn't give this property.

Thanks to @adriensamson for your help ;)