Fixed auto location of vpnkit socket

[yinzara]

Fixes #253

[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/moby/moby/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "master" git@github.com:yinzara/hyperkit.git somewhere
$ cd somewhere
$ git commit --amend -s --no-edit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.

[yinzara]

This PR maintains legacy behavior if the older versions of Docker for Mac are installed but supports the new location if the file exists.