Install trusty kernel from package manager

The version previously installed was used in lieu of the package manager
version due to it not being available in the package manager at the time.

The minimum version we now want is available in the package manager, so we're
returning to using the package manager.

Signed-off-by: Adam Stegman <astegman@pivotallabs.com>
Update OS_IMAGES.md

Signed-off-by: Adam Stegman <astegman@pivotallabs.com>