Upgrade to webpack-bundle-tracker to 1.0.0

[owais]

1.0.0-alpha.0 has been released and it only supports webpack 4 and newer. django-webpack-loader 1.0.0 should drop support for older webpack versions and rely on 1.0.0-alpha.0 of webpack-bundle-tracker. This mainly means updating the test suite and making sure it passes with webpack4 and bundle tracker 1.0.0

[owais]

WIP effort here: #217. Let me know if anyone would like to take it over.

[gilmrjc]

I've sent a PR #232 implementing the new version of webpack-bundle-tracker. It uses version alpha.1 and it handles the new stats format.

[rvlb]

We've merged #232. Release will follow soon.