chore(package): update gulp to version 4.0.1

Closes #50

Author: greenkeeper[bot]

package.json

@@ -73,7 +73,7 @@
     "eslint": "3.9.1",
     "eslint-plugin-react": "6.6.0",
     "expect-react-shallow": "~1.2.1",
-    "gulp": "3.9.1",
+    "gulp": "4.0.1",
     "gulp-gh-pages": "0.5.4",
     "mocha": "~3.1.2",
     "postcss-modules-extract-imports": "~1.0.0",