Move extension to require-dev as it is only needed for running tests

Author: zachborboa

composer.json

@@ -15,10 +15,10 @@
     "require": {
         "php": ">=5.3",
         "ext-curl": "*",
-        "ext-gd": "*",
         "ext-mbstring": "*"
     },
     "require-dev": {
+        "ext-gd": "*",
         "phpunit/phpunit": "*",
         "squizlabs/php_codesniffer": "*"
     },