Remove extra core utilised during docker deployment build

[wezrule]

The travis docker deployment cannot handle the additional core added in #2148, otherwise the error
virtual memory exhausted: Cannot allocate memory is given near the end of the build many times. It can handle the same number though, so I'm just removing the extra one. Using nproc instead now which is simpler than how I was determining the number of cores before.

For my sanity next time (and in case anyone else needs to do this), the following steps can be done to check the deployment build without have to merge into master:
1 - .travis.yml - Under stages, remove all except deploy. Under jobs -> include, remove all except deploy.
2 - ci/deploy-docker.sh - Replace this file with:

#!/bin/bash
set -e
docker build --build-arg NETWORK=live -f docker/node/Dockerfile -t any_name_you_want .