Add symfony/var-dumper dev dependency for easier development

Author: OndraM

composer.json

@@ -21,7 +21,8 @@
     "friendsofphp/php-cs-fixer": "^2.0",
     "squizlabs/php_codesniffer": "^2.6",
     "php-mock/php-mock-phpunit": "^1.1",
-    "satooshi/php-coveralls": "^1.0"
+    "satooshi/php-coveralls": "^1.0",
+    "symfony/var-dumper": "^3.3"
   },
   "autoload": {
     "psr-4": {