Use stable release of slevomat/coding-standard

[alcaeus]

This is a release blocker for 7.0: currently we require a dev-master commit of slevomat/coding-standard. This needs to be changed to a stable release before releasing 7.0.

[kukulich]

I was waiting for new release of PHPCS itself because of arrow functions support. Version 3.5.3 was released today, so I just need to write release notes for Slevomat CS and make the release :) This week for sure.

[alcaeus]

No worries - this issue is mainly here to block our automatic release tool: it refuses to create a release for a milestone with open issues. We've previously had to think of it and I'd like to have some insurance. Your release is ready when it's ready, we have some more things to take care of for 7.0 anyways.

[kukulich]

https://github.com/slevomat/coding-standard/releases/tag/6.0.0

[carusogabriel]

Fixed via #144.