Drop stage-0 babel preset

[shawnbot]

After reading this post, I'm pretty well convinced that we shouldn't be using stage-0 in our Babel presets. That stage contains a lot of different transforms, but (AFAIK) the only one we're actually using is object rest/spread. If that's true, then we should be much more explicit and only use plugins: ['transform-object-rest-spread']. In theory, it could also speed up our builds a bit by loading fewer plugins whenever babel is invoked.