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.

The only downside is that the 10.7 sdk is not easily separated from the rest of the dmg:
https://developer.apple.com/downloads/download.action?path=Developer_Tools/xcode_4.6.3/xcode4630916281a.dmg

That means, unfortunately, that a Mac is required to create a tarball from the MacOSX10.7.sdk directory in that dmg, which will likely not be a deterministic process. I don't believe that should affect the final output, though.

I would be very grateful if someone managed to crack open the dmg and extract the SDK from Linux, but none of the usual tools work on this one.