update slevomat/coding-standard dependency

Author: pueppiblue

composer.json

@@ -17,7 +17,7 @@
     ],
     "require": {
         "escapestudios/symfony2-coding-standard": "^3.5",
-        "slevomat/coding-standard": "^5.0",
+        "slevomat/coding-standard": "^6.0",
         "squizlabs/php_codesniffer": "^3.0"
     },
     "support" : {