On my system xmonad on Ubuntu 14.04, the xprop -root call has an earlier ocurence of NET_ACTIVE_WINDOW that makes this function fail. I guess someone who knows more about xprop should rewrite this function in a less brittle way, but for now my fix should do.
This patch will re-order the if statements when checking what operating
system the user is running.
This is useful for people that already have a `notify-send` binary in
their $PATH but are running OSX. An example of such requirements is
anyone running `vagrant-notify` is required to have a `notify-send`
binary on their $PATH.
See [link](https://github.com/fgrehm/vagrant-notify#os-x).
