add partprobing/partx calls on device when zapping

fixes a problem that would get devices in an inconsistent state after zapping.

The screenshot below shows how it picks `partx` or `partprobe` depending on the OS.


![screen shot 2014-07-03 at 2 34 24 pm](https://cloud.githubusercontent.com/assets/317847/3474189/b9011184-02e0-11e4-98df-617c519150e0.png)