bumping php-cs-fixer version as per comment from @erayd on jsonrainbow#563

Author: SignpostMarv

composer.json

@@ -32,7 +32,7 @@
         "icecave/parity": "1.0.0"
     },
     "require-dev": {
-        "friendsofphp/php-cs-fixer": "~2.2.20",
+        "friendsofphp/php-cs-fixer": "~2.15.1",
         "json-schema/JSON-Schema-Test-Suite": "1.2.0",
         "phpunit/phpunit": "^4.8.35"
     },