Travis: Update to latest LTS node versions, cache
Travis: Update to latest LTS node versions, cache

wincent/express-graphql,graphql/express-graphql,jamiehodge/express-graphql,wincent/express-graphql,jamiehodge/express-graphql,graphql/express-graphql,wincent/express-graphql,jamiehodge/express-graphql,graphql/express-graphql