chore(package): update gulp to version 4.0.2

Closes #56

Author: greenkeeper[bot]

package.json

@@ -32,7 +32,7 @@
     "exports-loader": "^0.6.3",
     "express": "^4.13.4",
     "file-loader": "^0.9.0",
-    "gulp": "^3.9.1",
+    "gulp": "^4.0.2",
     "gulp-bg": "^0.0.8",
     "gulp-changed": "^1.3.0",
     "gulp-changed-in-place": "^2.0.3",