Add php codesniffer as a dev dependency

Author: Daniel Gomes

composer.json

@@ -20,7 +20,8 @@
     "php": "^7.1"
   },
   "require-dev": {
-    "phpunit/phpunit": "^7"
+    "phpunit/phpunit": "^7",
+    "squizlabs/php_codesniffer": "^3.0"
   },
   "autoload": {
     "psr-4": {