React-styleguidist: Remove webpack 1 support

[sapegin]

This is a breaking change.

That’s a lot of work but most of the items are relatively easy and could be done separately, that’s why this issue is marked as easy — ping me if you have any questions.

Webpack 1 is deprecated in Styleguidsit (#499) — no complaints so far ;-)

• [x] Remove webpack conditions from make-webpack-config.js
• [x] Remove webpack conditions from .travis.yml
• [x] Remove warning from ensureWebpack function
• [x] Replace require.ensure with import()
• [x] Remove cacheable from loaders
• [x] Update webpack-dev-server
• [x] Update react-dev-utils
• [x] Remove json-loader from dependencies
• [x] Update min webpack version const
• [x] Disable ES modules in babel-preset-env
• [x] Update webpack-blocks instructions in the docs
• [x] Get rid of loaders package (?)
• [x] Reread migration guide ;-)

Do not remove support until:

• [x] Create React App supports webpack 2
• [ ] [webpack-blocks 1.0](https://github.com/andywer/webpack-blocks/tree/release/1.0) final release
• [x] [Webpack 3](https://github.com/webpack/webpack/releases) final release

[sapegin]

@andywer Any estimates on webpack-blocks 1.0 final release?

[andywer]

No concrete one, but we are quite close. Let's say about one more month maybe 😉

The release candidate will be out very soon and that's pretty much the deprecation of webpack-blocks 0.x.

[sapegin]

🍕 6.0.0 is out!!!! 🥓