chore(package): update gulp to version 4.0.1

Closes #18

Author: greenkeeper[bot]

package.json

@@ -24,7 +24,7 @@
     "babel-preset-es2015": "^6.3.13",
     "chai": "^3.4.1",
     "eslint": "^3.10.2",
-    "gulp": "^3.9.0",
+    "gulp": "^4.0.1",
     "gulp-babel": "^6.1.1",
     "gulp-eslint": "^3.0.1",
     "gulp-mocha": "^3.0.1",