Replace the canonical system-image server with the ubports system-image server. [Flo]

[ShadowEO]

UBports has brought up a few of the legacy devices, however some of these are not only under the ubports-touch suite, but also the legacy ubuntu-touch.

The Nexus 7 2013 is just one of the devices brought up under the
system-image.ubports.com/ubports-touch/15.04 URL.

I am simply proposing updating the URLs for the new system-image-server. I am going to play around with the JSON in the multirom manager installed on my tablet and will fork and submit a pull-request once I have figured out how the device headings work correctly. As last time I attempted this change to my own copy, MultiROMMgr was unable to find any channels. (It's something I'm doing wrong, I know it is.)