Offset live-reload port by default

This offsets the live-reload port by default. Subtracting 8000 gives us a different live-reload port than the default one (35729), but I don't know if this is what we want. Maybe we should always subtract `DEFAULT_PORT` instead, so the default live-reload port is being used if you don't change the port (which would be the case if you don't run multiple ember-cli instances)?