Bump default Compute Engine kernel

New kernel went out today together with new images using it, dated
0522 (prep work was last week). The kernel fixes security issues, which
I believe have already been publicly reported and fixed in Debian
(unsure). This makes future Compute Engine images use it by default
when images are added via build-debian-cloud.

The images were built with the previous commit, and the new kernel was
simply specified to gcutil addimage. No other changes to
build-debian-cloud were included, though Google's debs were revised to
improve network performance by configuring irq affinities via /proc at
startup. (We're planning to work on getting the necessary irqbalance
tweak into wheezy via stable-proposed-updates and then switch from
manual tweaks to installing irqbalance.)