fix(package): update gulp to version 4.0.1

Closes #100

Author: greenkeeper[bot]

package.json

@@ -32,7 +32,7 @@
     "autoprefixer": "^8.0.0",
     "css-loader": "^0.28.2",
     "extract-text-webpack-plugin": "^3.0.2",
-    "gulp": "^3.9.0",
+    "gulp": "^4.0.1",
     "gulp-concat": "^2.6.0",
     "gulp-postcss": "^7.0.0",
     "gulp-sass": "^3.1.0",