Add last stable version in `docker version`

I would like to add le last stable version in `docker version`
The client will use the Github API to get the last version available (using tags)


I think it would save us some time, often we ask users what is their version, and then we tell them this bug was fixed in the last version. I think if they see `please update docker` in `docker version` we will save everybody some time.

    $> docker version
    Client version: 0.4.8
    Server version: 0.4.8
    Go version: go1.1.1
    Last stable version: 0.4.8

or

    $> docker version
    Client version: 0.4.8
    Server version: 0.4.7
    Go version: go1.1.1
    Last stable version: 0.4.8, please update docker