Update to babel 7

[DeMoorJasper]

There has been some changes in the typings package. So I updated that package to v7.
While I was at it, I also added an engines field in package.json and updated the internally used babel to 7 so builds go slightly faster and benefit from all new bugfixes.

This is a breaking change, as support for Babel 6 will be dropped.

[DeMoorJasper]

@TimothyGu When will this be pushed to npm?

[joaovieira]

Yes, please. Can we publish it? We need this for JSX's new <> fragment syntax.

[joaovieira]

ping @TimothyGu ☝️

[joaovieira]

@TimothyGu, again, can we publish this? It's a dependency of https://github.com/fraserxu/babel-jsxgettext and without this PR we can't parse JSX trees that include the new Fragment syntax <>. It's causing many headaches that could be avoided if we just published this.

[AxelDelmas]

Same here. Can you publish it on npm, please?

[AxelDelmas]

I made the builds available on github, if you're blocked you can change your package.json to install babylon-walk from this repo: https://github.com/streamroot/babylon-walk (this fork won't be maintained at all though, it's just a temporary solution before the package is updated on npm)