Update node.js version in docker image

The docker image includes node.js version 0.10.29, which is too old to run the current set of JS unit tests. The latest stable version is 5.1.0 and the "mature and dependable" version is 4.2.2, so the version in docker (from ubuntu) is quite old. The install instructions for 4.2.2 aren't very difficult:

curl -sL https://deb.nodesource.com/setup_4.x | sudo -E bash -
sudo apt-get install -y nodejs