Upgrade to Babel 7

[alexlafroscia]

As talked about in #75, this drops Babel 6 support entirely in favor of Babel 7

[alexlafroscia]

I've been digging into this issue all morning and haven't made any headway yet. Partly for my own sake, I'll leave some notes:

• In production builds, something is adding both the decorators and decorators-legacy babel plugins. This only happens in production builds, not development builds
• While debugging, it seems that the Babel plugins being run on each file between production and development are entirely different. There are plugins included in the production builds that are being included from a currently-unknown source. I'm trying to track down where they are being included from
• @ember-decorators/babel-transforms, which provides the decorator transform to this addon, does not itself have this same issue. I am not sure why that would be, since @ember-decorators/argument does not configure the decorator transforms in any way
• The set of plugins being run is more than just those defined in parent.options.babel.plugins

[alexlafroscia]

Aha! babel-plugin-filter-imports is the culprit

https://github.com/ember-cli/babel-plugin-filter-imports/blob/f2b57e1a7a26091be5389745ea085aef6d3c4869/src/index.js#L9

@pzuraq you added the original inclusion of the decorators plugin to that -- why does the import filter care about decorators?

https://github.com/ember-cli/babel-plugin-filter-imports/commit/69fd4359d41240469e2cb7062d3dc3cd860de948

[alexlafroscia]

I opened this PR on babel-plugin-filter-imports in hopes of moving this along.

https://github.com/ember-cli/babel-plugin-filter-imports/pull/64

[alexlafroscia]

With the update to babel-plugin-filter-imports that was just released, I think this should actually be ready to go!