coreos-kernel: roll back to previous 3.15.8 kernel

The new kernel completely failed to boot on EC2 as a Xen PV instance.
Rolling back until we figure out what is going on.

Boot log: https://gist.github.com/marineam/50f92ded40e02c163c19
Merge pull request #834 from marineam/revert

coreos-kernel: roll back to previous 3.15.8 kernel