fix(package): update gulp to version 4.0.2

Closes #9

Author: greenkeeper[bot]

package.json

@@ -2,7 +2,7 @@
   "name": "developer.github.com",
   "version": "0.0.1",
   "dependencies": {
-    "gulp": "^3.9.0",
+    "gulp": "^4.0.2",
     "gulp-coffee": "^3.0.2",
     "gulp-concat": "^2.6.0",
     "gulp-if": "^2.0.0",