change from using port 4243 to 2375 - this is incompatible with previous Boot2Docker ISO's

While I could wire up both ports on the VM, we would then need 2 ports on the osx box, add cfg and so on, all of which we don't want.

I'll contemplate making b2d-cli ssh to the container to get the /etc/version file and fall back, but more testing will, er, suck.