Bugfix: Run aptly in screen until 0.9 is released

gpg wont work with aptly 0.8 in cronjobs, it fails with `gpg: cannot open `/dev/tty': No such device or address`. There is already a fix in aptly (https://github.com/smira/aptly/pull/122) which will be available in the upcoming version 0.9. In the meantime, this PR runs the aptly script in a headless screen instance during the cronjob, so gpg has /dev/tty and is happy.