NzbDrone v1533 - Added SPK compatibility for built-in NzbDrone updates!

SPK now versioned by date for multi-branch and built-in NzbDrone updater compatibility - SPK upgrade now retains current NzbDrone binary (avoids potential binary downgrade) when necessary
Would it make sense to set this to `SPK_VERS = $(shell date +%Y%m%d)`? This sets `SPK_VERS` to the package build date, same as for e.g. sickbeard-custom package.
If not, let's lose the dots, so it's at least the same format as those packages.