Use babel to transpile the workbox-build codebase?

**Library Affected**:
*workbox-build*

**Browser & Platform**:
*all browsers*

**Issue or Feature Request Description**:
Currently making a PR, should be as easy as throwing in a 'use strict';

Ref: https://travis-ci.org/thelounge/lounge/jobs/251861458
and thelounge/lounge#1317