kpatch Ubuntu support and more

See issue #156

On Ubuntu 14.04 LTS
* `kpatch load` failed to insert the kpatch core module and the patch module
* `kpatch unload` failed to remove the patch module

Because `kpatch` script assumes that  `insmod` and `rmmod` reside in `/usr/sbin` whic is NOT the case for Debian and Ubuntu (in `/sbin`).

> NOTE: On Arch Linux, `kmod` installs `insmod` and `rmmod` to `/usr/bin`. Could be more cases for different distros.

The patch is the simplest change I can think of which makes the script distro independent.