coreos-install: include new signing subkey

The previous subkey that signed images up until now has expired.

pub   4096R/93D2DCB4 2013-09-06
uid       [ unknown] CoreOS Buildbot (Offical Builds) <buildbot@coreos.com>
sub   4096R/74E7E361 2013-09-06 [expired: 2014-09-06]
sub   4096R/E5676EFC 2014-09-08 [expires: 2015-09-08]
Merge pull request #128 from marineam/subkey

coreos-install: include new signing subkey