Upgrade to Babel 7

[pioug]

I did the upgrade to Babel 7.0.0-beta.3.

I removed the arrayify functions. It's a breaking change but it's aligned with https://babeljs.io/blog/2017/03/01/upgrade-to-babel-7#option-parsing if I understand correctly.

So if anyone has more insight 🙏 I'd like to know so I can update the PR.

I remove the tests for Node <= 0.12 since it is not supported anymore https://babeljs.io/blog/2017/03/01/upgrade-to-babel-7-for-tool-authors#all-babel-packages-node-js-support

UPDATE: Just found a similar PR #242 😶

[oztune]

Would it make sense to make babel-core a peerDependency? This way at least minor versions can be upgraded independently of this package. tsify does this and it seems to work for them.

[pioug]

@oztune Good point! Just checked babel-loader does something similar https://github.com/babel/babel-loader/blob/master/package.json#L18-L19

Thanks for reviewing 🙏

[oztune]

@hzoo Do you think this is ready for use with babel@7.0.0-beta.x?

[pioug]

Closing, see #255 instead.