chore(*): bump CoreOS to 494.1.0 for Docker vulnerability

This PR updates CoreOS to include [Docker 1.3.2](https://github.com/docker/docker/blob/master/CHANGELOG.md#132-2014-11-20) which includes [important security fixes](https://groups.google.com/forum/#!msg/docker-user/IrjXTHA6jJc/ZToMGL2yG_AJ). In addition to upgrading to Docker 1.3.2, we are addressing the secure-by-default registry settings in #2578 (these changes were introduced in Docker 1.3.1). Also, we switch to the beta channel for CoreOS since we are no longer waiting for upstream CoreOS changes to land for Deis (i.e. CephFS kernel support, etc.). We hope to track the stable channel once this build makes it to stable.

Note that DigitalOcean and Rackspace do not yet have CoreOS 494.1.0.