[REQ] Update PHP Slim generator to 4.0.0-alpha

Created by: ybelenko

Is your feature request related to a problem? Please describe.

Slim Framework announced 4.0.0-alpha release in previous month. https://www.slimframework.com/2019/04/25/slim-4.0.0-alpha-release.html Current Slim server generator based on 3 version.

Describe the solution you'd like

I think it's perfect time to migrate to Slim 4 while openapi-generator is in prestable status.