Uses babel-preset-env to transpile our bundled code

[jeffposnick]

R: @philipwalton @addyosmani @gauntface
CC: @poshaughnessy @HenrikJoreteg

Fixes #812

I'm hopeful that this should lead to compatibility with Samsung Browser 5, as it's based on Chrome 51. Assuming this is merged, we can publish a pre-release tag of v2.0.2-alpha if folks want to test against that in the real world and confirm compatibility prior to the official release.

While we're touching the Babel configs, I took the opportunity to switch from the deprecated babili package to its current incarnation, babel-minify.

The final, gzipped payload size of the workbox-sw.prod.js bundle is 14,560 bytes after applying the transpilation, which is an increase of only ~260 bytes vs. the previous 2.0.1 bundle.

[addyosmani]

Thanks Jeff. Diff LGTM.

Still need to verify this works on the version of Samsung Browser that we were previously breaking on.

[philipwalton]

LGTM

I think there's probably some optimizing we could do here with the external helpers whitelist, but it may not be worth experimenting with that until v3.

[jeffposnick]

The CI failures have apparently been cropping up in the daily cron runs of Travis as well with the current master, and are due to timeouts in Firefox. Here's one from yesterday unrelated to this PR:

https://travis-ci.org/GoogleChrome/workbox/jobs/278383221#L1673

I manually deployed an instance of the transpiled workbox-sw code to https://ifixit-pwa.appspot.com/ and tested it out before and after with Samsung Browser (v5.4, I think), and confirmed that it starts working offline with the transpiled code. There doesn't appear to be any regressions when I manually tested in other browsers, either.

I'd still like to tag this as pre-release on npm initially to get confirmation from other folks. I'll update the underlying GitHub issue thread when it's published.

[HenrikJoreteg]

Thanks for getting this sorted out folks 🎉