Allow publishing of externally built tarballs

[juliushaertl]

I just tried to release a nightly build of deck to the app store, but if fails with the following error message:

$  krankerl publish --nightly https://download.bitgrid.net/nextcloud/deck/nightly/deck_nightly_2017-12-07.tar.gz
an error occured: Other("No package found")

@ChristophWurst