Yosemite compatibility

This should fix https://github.com/chrishunt/desktop/issues/17

Yosemite is apparently caching the desktop image. This will `touch` the desktop image before reloading it so that Yosemite knows that it has been updated.

![6a0120a85dcdae970b0128776ff992970c-pi](https://cloud.githubusercontent.com/assets/65323/4686718/4f408dc6-5649-11e4-90a1-376d0b31ccba.png)