Use nodejs build matrix

[bekicot]

This is a node project, it must have node as its global language.
That is needed so that we can have multiple versions in the global matrix.

[jayvdb]

This project is not a reusable app, so the importance of supporting multiple node versions is much lower, but this is still useful to do as it shows GCI high school students how to do Node projects properly in CI.

coala/gh-board#59 is prior art, and I did the same for git-task-list.

However this should considered part of https://gitlab.com/coala/mobans/issues/34 , to standardise all our node static website projects. The most important part is security of the deploys.