travis: Install latest npm in before_install
travis: Install latest npm in before_install

Something funny happened with the package-lock.json format between npm
versions 5.0.0 and 5.0.1. This should resolve the current breakage
resulting from this discrepancy:

https://travis-ci.org/mbland/url-pointers/jobs/238342590

mbland/url-pointers,mbland/custom-links,mbland/url-pointers,mbland/custom-links,mbland/custom-links,mbland/url-pointers