"10.9" was the wrong `minSupportedOSVersion` for preventing OS X 10.7.x
and 10.8.x users from getting updated to Tor Browser 7. That it worked
for OS X 10.6.x was only by chance according to the release history table
(https://en.wikipedia.org/wiki/Darwin_(operating_system)#Release_history).

We fix that by choosing "13.0.0" (for OS X 10.9 as minimum).