Add phpstan as static code analysing tool as dev dependency

[marcelthole]

And here is the dev dependency for phpstan.
I split it, because i didn't know if you would add a new package in your repository
/see #2522

[marcelthole]

oh sorry. There is already a phpstan PR #2505 - my fault

[coveralls]

Coverage remained the same at 97.802% when pulling 4c872cd on marcelthole:add-phpstan-dependency into 5b56754 on slimphp:4.x.

[akrabat]

PHPStan support has landed via a different PR. Thanks for helping Slim though!