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.

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.)