Update tests to webpack 4 + babel 7

[prayerslayer]

Hi!

I want to see splitChunks support here soon, so I figured I'll update tests to webpack 4 :)

Changes are mostly mechanical:

Updated packages

• New versions for babel, webpack, react etc.
• Replaced extract-text-css-plugin with mini-css-extract-plugin because that works in our project and there was a time where the former didn't work with webpack 4. I didn't check if that's still the case.
• Removed webpack-split-by-path package, test + examples, because it's quite old (2 years) and webpack has splitting built-in now.

Updated webpack configs

Since I updated from webpack 1 to 4 the format of a webpack config changed a bit.

Updated test cases

They're not different, functionally speaking. styles.css became main.css because it's named as the bundle. The output order of the js and css chunk changed, but imo that shouldn't have been tested in the first place :) Other than that it's the same.

[prayerslayer]

not sure what's wrong with travis here :/

[fjsj]

Thanks for your contribution! We're reorganizing the project now.

@rvlb those commits are also at #261

Do you rather fix and merge this one first or focus on fixing 261?

[fjsj]

I believe those behaviors are already tested on current master. If they are not, please let us know.