Add unix socket support

@nahiluhmot @bfulton 

Per issue #40, added support for running Docker with the Unix socket.  I made this the default behavior because this is now Docker's default behavior.

To continue using the port, use `Docker.with_port` during initialization.

This does require using Excon version 0.27.0.

One thing to recognize is that we shouldn't ever be hand editting a VCR cassette.  Always rerun VCRs since things got missed that should've made tests fail.