* Updates ember-cli itself to use babel@6.
* Removes usages of `options.babel` for internal babel version.
* Removes `_pruneBabelOptions` method.

* Use private internal babel options for methods using ember-cli
  shipped babel transpiler.
* Provide babel options based upon ember-cli-babel version (for
  babel@5 give babel@5 style options, for babel@6 give babel@6
  style options).

Provide `this.options.babel` based on

* Remove usage of `babel-preset-env`.
* Change all "fallback" processing to handle **only** modules.

When using ember-cli-babel@5 use babel@5 options, when using
ember-cli-babel@6 no defaults are needed.

Fixes an issue with using babel@6 options when transpiling with
babel@5.

The RFC has landed, and ember-cli-babel has been updated to take
advantage of it. Having real use cases was the blocker to enabling
the feature flag and removing it.

babel@6.24 includes the required interop changes.

Fixes some bugs.