Image size limit

[Tim]

What do people think about increasing the size limits on our ISO's, we currently have a 1GB limit set, but that is pretty arbitrary and was
mainly intended so the image would fit on 1GB usb key. That seems like a somewhat dated requirement these days, with usb keys much cheaper and
for the most part probably much bigger than 1GB.

15.04 (vivid) images, just broke the limit, but the only way to get them under would have been to remove the kde atk (accessibilty) libs, which
the debian maintainer is dead set against, so we didnt.

This cycle we be adding the getting start tour and probably a few other bits, so seems like a hard task to get back under 1GB again. fwiw Ubuntu
Vanilla was also 1.1GB for vivid.