fix: require @babel/preset-env instead of babel-preset-env

[buschtoens]

We're listing @babel/preset-env as a dependency and so should be using it. 😄

This was the reason for transpilation failing on me for IE11, because some old incompatible transforms are used by babel-preset-env.

AFAICT I can now fully build our TypeScript app with just ember-cli-babel. 😍 🎉

Simple demo repo here: https://github.com/buschtoens/ember-typescript-babel-7

Thank you guys so much. ❤️

Closes #236, because it branched off of it.

[buschtoens]

One more thing: I added @babel/core@^7.0.0-beta.56 as a dev dep to fix the missing peer dependency. But it should probably be a real dependency, right? Because right now the host app needs to install @babel/core for us. Is this intentional?

[rwjblue]

Because right now the host app needs to install @babel/core for us.

No, broccoli-babel-transpiler has a direct dependency on @babel/core.

[rwjblue]

Can you rebase (@dfreeman got CI passing in #239)?

[buschtoens]

No, broccoli-babel-transpiler has a direct dependency on @babel/core.

True! Also noticed that. The problem for me was, that some of my transforms had a peer dep on ^beta.56, but broccoli-babel-transpiler has 42 or something like that.

So I guess this PR is outdated now. I'll rebase #236 instead.