chore(package): update gulp to version 4.0.2

Closes #12

Author: greenkeeper[bot]

package.json

@@ -43,7 +43,7 @@
   },
   "devDependencies": {
     "chai": "^3.5.0",
-    "gulp": "^3.8.11",
+    "gulp": "^4.0.2",
     "gulp-jscs": "^4.0.0",
     "gulp-jshint": "^2.0.4",
     "gulp-mocha": "^3.0.1",