Support for PHPCS ^4.0

[KminekMatej]

Currently installing doctrine/coding-standard is not possible if we are using on root project squizlabs/php_codesniffer: 4.0.x-dev

[greg0ire]

I don't think we should add support for unstable versions of our dependencies. Why go through the CI complications that will induce when we could wait for a stable version?

[KminekMatej]

@greg0ire You're right, somehow I missed the dev appendinx, my bad. Closing