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.
Merge pull request #87 from Geoportail-Luxembourg/update-ngeo

Add an update-node-modules Makefile target