release: Bump the OSX SDK to 10.7 for gitian builds

This fixes the display on Retina Macbooks. It also moves us away from depending
on the ancient XCode3 sdk.

Conflicts:
	doc/release-process.md

Rebased-By: Wladimir J. van der Laan <laanwj@gmail.com>
Rebased-From: 2869b13