Fix the mini-css-extract-plugin 0.4.3 issue and increase min. allowed Webpack version

[Lyrkan]

This PR fixes both of the issues detected in #403:

• Set the maximum allowed version for mini-css-extract-plugin to 0.4.2 (since 0.4.3 doesn't work with the current version of the webpack-manifest-plugin, see Incompatibility between mini-css-extract-plugin 0.4.3 and the manifest plugin #404). This is a temporary solution, once it's fixed (either in mini-css-extract-plugin or in weback-manifest-plugin) we'll have to change it again.

• Set the minimum allowed version for webpack to 4.20.0. The previous one (4.0.0) seemed to break mini-css-extract-plugin (see https://travis-ci.org/Lyrkan/webpack-encore/jobs/439810002).

[Lyrkan]

By the way, once we update mini-css-extract-plugin to 0.4.4+ we'll also have to change some of the tests related to versioning due to this commit (hashes not being the same anymore).

[weaverryan]

Looks like a bit of a mess with mini-css-extract. 0.4.4 is now out. Does it fix the issue, or does the issue remain? Another way of asking this is: will the issue be fixed in mini-css, or is their opinion that this is not a bug, and it should be fixed elsewhere?

[Lyrkan]

Does it fix the issue, or does the issue remain?

That's still an issue when using 0.4.4: https://travis-ci.org/symfony/webpack-encore/jobs/439825872#L1055

The only different thing with 0.4.4 is that more tests are failing because it changes how some hashes are generated.

will the issue be fixed in mini-css, or is their opinion that this is not a bug, and it should be fixed elsewhere?

Based on the last comments here I'm not sure that there is an issue with mini-css-extract-plugin.

[weaverryan]

Hmm ok. Obviously, this is far from ideal :). I created #415 to track this. My guess is that we're going to need to go into the manifest plugin ourselves and submit a PR to fix this - I noticed one +1 on it, but no feedback :/

Anyways, thanks! This fixes our immediate problem - which is great.