travis: Use phantomjs from the node_modules cache
travis: Use phantomjs from the node_modules cache

Per https://www.npmjs.com/package/phantomjs-prebuilt, this should allow
Travis to use PhantomJS from the build cache rather than downloading the
binary with every build.

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