tweak apt::source declaration

* Set release parameter correctly
* Get public key from HTTPS site to reduce chance of MITM attack
Remove unneeded exec

As the license key is set in the nrsysmond.cfg template there's no need to
use an extra command to set it - the file is already generated before the service
is notified.
Merge pull request #19 from 3flex/patch-3

Remove unneeded exec
Merge pull request #18 from 3flex/patch-2

tweak apt::source declaration