Adjust kernel version parsing to be more lenient of strange things like "3.12-1-amd64"

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #4316 from tianon/more-lenient-kernel-version

Adjust kernel version parsing to be more lenient of strange things like "3.12-1-amd64"