ignore `node_modules`

The `.gitignore` file tells git which files it should not pay attention
to.

Because Heroku automatically runs `npm install` and installs our
dependencies listed in `package.json`, we don't want to save them along
with our app, so we ignore the files in `node_modules`.
Add express

`npm install express --save`
Upgrade our server to use express

Notice how much less code we need when we use express.