fix(package): update feed to version 4.0.0

Closes #6

Author: greenkeeper[bot]

package.json

@@ -39,7 +39,7 @@
     "node": ">=8"
   },
   "dependencies": {
-    "feed": "2.0.4",
+    "feed": "4.0.0",
     "lodash.defaultsdeep": "4.6.0",
     "lodash.isempty": "4.4.0",
     "remove-markdown": "0.3.0",