[Shotwell] Change in PPA structure

[Jens Georg <mail jensge org>]

Hi,

as you might have seen from the announcements that I have created a
second PPA that will contain all unstable releases so that you can
decide to stay on the stable releases or follow the unstable releases:

Stable PPA:
https://launchpad.net/~yg-jensge/+archive/ubuntu/shotwell

Unstable PPA:
https://launchpad.net/~yg-jensge/+archive/ubuntu/shotwell-unstable