Wrong cache directory in shared backend

[gcampax]

All engines in the shared backend use the same cache directory /srv/thingengine/cache, but they each initialize it's own ModuleDownloader which will try to download skills into it. This can cause issues (race conditions) when skills are upgraded.
It's likely the cause of stanford-oval/thingpedia-api#185 where one device was too new in the cache (version higher than the version available in thingpedia)