ci: drop the `node_modules` caching on Travis
ci: drop the `node_modules` caching on Travis

Travis runs `npm ci` when a `package-lock.json` file is present, and 
that fails for various reasons when the `node_modules` folder is cached. 
As `npm ci` performs a clean install always, caching the `node_modules` 
folder does not make sense anyway.

tediousjs/tedious,pekim/tedious,tediousjs/tedious