fix(package): update gulp to version 4.0.2

Closes #301

Author: greenkeeper[bot]

package.json

@@ -45,7 +45,7 @@
     "fs-extra": "^5.0.0",
     "glob": "^7.0.3",
     "glob-promise": "^3.3.0",
-    "gulp": "^3.9.1",
+    "gulp": "^4.0.2",
     "gulp-autoprefixer": "^4.1.0",
     "gulp-babel": "^7.0.0",
     "gulp-concat": "^2.6.1",