Between 18:00 and 21:20 UTC, OSX builds which use our Xcode 6.4 image were failing to boot, due to the accidental deletion of the image during some maintenance clean up of our image catalog. As soon as we realized the error we built a new 6.4 image, which has since been published and is verified as working. Please email support@travis-ci.com if you see any issues with Xcode 6.4 builds.