bump slevomat/coding-standard to fix CI

[orklah]

This will fix the CI crash

[orklah]

...except we can't run that on PHP 7.1.

I'll try to fix that another way. The core issue seems to come from phpstan/phpdoc-parser in version 1.7. We could pin the previous version on 4.x

[weirdan]

@orklah there's already a number of packages removed on PHP 7.1, just add slevomat/coding-standard to that list. CS checks do not run on 7.1 anyway.

[orklah]

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