Add -o pipefail to bash when checking kernel version

[twelho]

This enables bash to return an error if any of the commands in the
pipeline fail (such as when the strings binary is missing).
Fixes #181.

[twelho]

FYI pipefail also works with ksh and zsh in addition to bash, so this solution is quite universal. Requiring the kernel version to be numeric or match a regexp is not a good solution in my opinion, as custom kernels can have multiple string tags in the version etc.

[luxas]

Thanks! This works equally well IMO 👍

LGTM