Add support for client binary on upgrade

Went ahead and implemented this, looking for feedback, how do those file permissions look to you?

ping @SvenDowideit @tianon 
I'm still not understanding why `arch = "windows"` here... :wink:

https://github.com/boot2docker/boot2docker-cli/releases has `boot2docker-v1.4.0-windows-amd64.exe`