Cannot find module 'sass'

```js
'use strict';

var sass = require('sass');
const EmberApp = require('ember-cli/lib/broccoli/ember-app');

module.exports = function(defaults) {
let app = new EmberApp(defaults, {
// Add options here
sassOptions: {
implementation: sass
}
});

// Use `app.import` to add additional libraries to the generated
// output files.
//
// If you need to use different assets in different
// environments, specify an object as the first parameter. That
// object's keys should be the environment name and the values
// should be the asset to use in that environment.
//
// If the library that you are including contains AMD or ES6
// modules that you would like to import into your application
// please specify an object with the list of modules as keys
// along with the exports of each module as its value.

return app.toTree();
};
```

➜ flexbox-app git:(master) ✗ ember s
Could not require 'ember-cli-build.js': Cannot find module 'sass'

Stack Trace and Error Report: /var/folders/1c/5t4__m4j72zfnrchkthtmh8c0000gn/T/error.dump.10b05e33237a0aabe49205f7efb7b06c.log