chore(package): update gulp to version 4.0.2

Closes #56

Author: greenkeeper[bot]

package.json

@@ -71,7 +71,7 @@
     "css-loader": "^1.0.0",
     "cssnano": "^4.1.10",
     "del": "3.0.0",
-    "gulp": "3.9.1",
+    "gulp": "4.0.2",
     "gulp-bump": "3.1.1",
     "gulp-conventional-changelog": "^2.0.10",
     "gulp-coveralls": "0.1.4",