chore(ci): drop AppVeyor for Windows build on Travis

[Kocal]

AppVeyor failed on #732, but then it goes green without anyone doing anything 🤷‍♂️

• failing build: https://ci.appveyor.com/project/fabpot/webpack-encore/builds/32261340/job/1ndms5u5vx74hr21#L844
• and then it's green: https://ci.appveyor.com/project/weaverryan/webpack-encore/builds/32261339

Apparently it's not the first time AppVeyor is weird:

What abouy removing it and let Travis run a Windows build? Unfortunately only arch amd64 is supported, see Travis documentation.

What do you think?

[nicolas-grekas]

I followed what the support forum tells to do in case this error happens.
https://help.appveyor.com/discussions/problems/11287-the-build-phase-is-set-to-msbuild-mode-default-but-no-visual-studio-project-or-solution-files-were-found

I'd prefer continuing to use appveyor. More providers means more concurrent jobs.

[Kocal]

Great!

So I'm closing this.

[Kocal]

The issue happens again on #736:

• is red: https://ci.appveyor.com/project/fabpot/webpack-encore/builds/32281343
• then suddenly it's green: https://ci.appveyor.com/project/weaverryan/webpack-encore/builds/32281342

What do you think about using GitHub Actions?