Update package.json

- Upgrade to gulp 4 for local development
- Update lodash and react-docgen
- Remove gulp-util dependency

Author: cassiostp

package.json

@@ -24,16 +24,15 @@
     "url": "https://github.com/adroll/gulp-react-docs/issues"
   },
   "dependencies": {
-    "gulp-util": "^3.0.7",
     "handlebars": "^4.0.5",
-    "lodash": "^3.10.1",
-    "react-docgen": "^2.20.0",
+    "lodash": "^4.17.15",
+    "react-docgen": "^5.3.0",
     "through2": "^2.0.0"
   },
   "devDependencies": {
     "del": "^2.2.0",
     "doctoc": "^0.15.0",
-    "gulp": "^3.9.0",
+    "gulp": "^4.0.2",
     "gulp-concat": "^2.6.0",
     "gulp-load-plugins": "^1.1.0",
     "gulp-tap": "^0.1.3"