Allow usage of async/await

[topaxi]

Every browser with SW support, supports async/await too.

This PR removes regenerator from the babel options and updates broccoli-rollup and broccoli-uglify-sourcemap to be able to parse async/await syntax.

[ghost]

Looks great! But does it not crash on a registration if you use async/await in an unsupported browser?

[bcardarella]

If it does I'm in favor of a progressive enhancement path rather than bringing in regenerator. The footprint of that plugin is not small.

[topaxi]

This will defintely "crash and burn" if one uses async/await in sw-registration code in browsers without SW support. It would crash anyways today as we'd need to import regenerator explicitly (I'm not sure if ember-maybe-import-regenerator works here, never tried).

So this is only a problem if one uses async AND ships code for non-SW browser in registration code (which might not be that uncommon? I don't know).

Maybe implement this as a flag? Or the ability to customize babel options for ember-service-worker?

// ...
module.exports = function(defaults) {
  let app = new EmberApp(defaults, {
    'ember-service-worker': {
      babel: { blacklist: ['regenerator'] },
    },
  });
  // ...
};

[ghost]

I like that idea, to supplement the babel options if possible.