chore(package): update gulp to version 4.0.1

Closes #15

Author: greenkeeper[bot]

package.json

@@ -58,7 +58,7 @@
   "devDependencies": {
     "dustjs-linkedin": "^2.6.x",
     "expect.js": "^0.3.0",
-    "gulp": "^3.8.0",
+    "gulp": "^4.0.1",
     "gulp-develop-server": "^0.5.2",
     "gulp-expect-file": "^0.0.7",
     "gulp-istanbul": "^1.1.1",