Replace es3ify with Babel ES3 transforms

[gaearon]

The only reason it was added is because Babel ES3 transforms were broken.
They have since been fixed.

Now that es3ify itself is broken (see Travis), I’d appreciate somebody just putting those plugins in and removing es3ify from our build process.

[chentsulin]

Submitted a PR at #1688

[gaearon]

Thank you!