Xcode 7.2.1 was recently released and fixes an annoying bug that would prevent the simulator from launching if your app needs longer than 120 seconds to build. You kindly upgraded to Xcode 7.3. I think it should be safe to upgrade the existing 7.2 machines to 7.2.1.
@levlaz@pietbrauer We already pre-install Xcode 7.2.1 in the containers. Please let me know if you see behaviour that does not reflect that in your builds.