Upgrade to babel 7

[dantman]

Babel 7 is significantly better than Babel 6. When it is released or at least gets close enough to release we should switch to the new version.

We may even be able to make use of @babel/preset-env's new useBuiltIns: 'usage' to only import polyfills for things we are using. babel-runtime is including the polyfill for Promise even though Node 6.x doesn't need it.