Update dependency load-grunt-config to v3

Author: renovate-bot

package.json

@@ -41,7 +41,7 @@
     "eslint-plugin-yoast": "^1.0.1",
     "grunt": "^1.0.1",
     "grunt-eslint": "^20.1.0",
-    "load-grunt-config": "^0.19.2",
+    "load-grunt-config": "^3.0.0",
     "nightwatch": "^0.9.15",
     "ps-tree": "^1.1.0",
     "time-grunt": "^1.4.0",