fix(package): update gulp to version 4.0.1

Closes #20

Author: greenkeeper[bot]

package.json

@@ -23,7 +23,7 @@
   },
   "dependencies": {
     "foundation-sites": "^6.3.1",
-    "gulp": "^3.9.0",
+    "gulp": "^4.0.1",
     "gulp-autoprefixer": "^5.0.0",
     "gulp-concat": "^2.6.1",
     "gulp-load-plugins": "^1.1.0",