chore(package): update gulp to version 4.0.2

Closes #120

Author: greenkeeper[bot]

package.json

@@ -54,7 +54,7 @@
     "eslint-config-strict": "^14.0.0",
     "eslint-plugin-import": "^2.7.0",
     "estraverse-fb": "^1.3.1",
-    "gulp": "^3.9.1",
+    "gulp": "^4.0.2",
     "gulp-babel": "^7.0.0",
     "gulp-eslint": "^4.0.0",
     "mocha": "^3.4.2",