using node source for node dockerfile

[dkebler]

fyi nodsource now supports arm so this works very nicely now as opposed to getting the tarball.
This might be perferable. Nodesource is well established so it's a "safe" alternative to grabbing the tarball directly and will look for arm 6,7,8 and load appropriately.

curl -sL https://deb.nodesource.com/setup_7.x | bash -
apt-get install -y nodejs

[alexellis]

Good suggestion. Please send a PR, just add a new folder for that version 👍