Require ext-gd for development

Author: jjok

composer.json

@@ -65,6 +65,7 @@
     },
     "require-dev": {
         "ext-zip": "*",
+        "ext-gd": "*",
         "phpunit/phpunit": "^4.8.36 || ^5.0",
         "squizlabs/php_codesniffer": "^2.9",
         "friendsofphp/php-cs-fixer": "^2.2",