cache travis node modules
cache travis node modules

This will speed up builds by several minutes, but we will have to clear
the cache when dependencies are updated.
See https://docs.travis-ci.com/user/caching/#Clearing-Caches

diasdavid/browser-laptop,luixxiul/browser-laptop,diasdavid/browser-laptop,timborden/browser-laptop,luixxiul/browser-laptop,timborden/browser-laptop,luixxiul/browser-laptop,diasdavid/browser-laptop,timborden/browser-laptop,luixxiul/browser-laptop,diasdavid/browser-laptop,timborden/browser-laptop