chore(package): update gulp to version 4.0.2

Closes #69

Author: greenkeeper[bot]

package.json

@@ -39,7 +39,7 @@
     "es6ify": "^1.6.0",
     "eslint": "^5.0.0",
     "expect.js": "^0.3.1",
-    "gulp": "^3.9.0",
+    "gulp": "^4.0.2",
     "gulp-connect": "^5.6.1",
     "gulp-jshint": "^1.11.2",
     "gulp-uglify": "^3.0.1",