Add an update-node-modules Makefile target

Currently to update ngeo (or openlayers) to the latest "master" one must manually remove the "node_modules/ngeo" directory first (`rm -rf node_modules/ngeo`). This PR adds an "update-node-modules" target that does `npm install --force`. This forces the re-installation of all the node dependencies, which will effectively download the latest "master" for ngeo.

Please review.