Update Babel config

[taion]

No description provided.

[taion]

Making use of https://github.com/babel/babel/releases/tag/v6.13.0 to simplify our Babel configuration considerably.

[timdorr]

Neat. But I'm not sure I like the indirection to another file somewhere else. Why not just keep the envs and declare the options that way? Less code, more config.

[taion]

The main downside of doing it that way is repeating "es2015-{whatever}", "stage-1" everywhere, and ending up with potentially broken config otherwise.

The other downside is that, because we can't apply the standard es2015 transform, as configured, unless you explicitly set the environment, you can't actually run any of our code.

[taion]

@timdorr Do you feel strongly that we should get rid of the custom es2015 preset with the BABEL_ENV conditional?

[timdorr]

Not so much that it really matters. Let's just do it.

[taion]

Okay, thanks. I'm going to try to make this a pattern in a bunch of places, so hopefully it will be less unfamiliar.