chore(package): update mqtt to version 4.0.1

Closes #21

Author: greenkeeper[bot]

package.json

@@ -30,7 +30,7 @@
     "coffee-script": "^1.10.0",
     "mocha": "^5.0.0",
     "amqplib": "^0.5.1",
-    "mqtt": "^2.13.0",
+    "mqtt": "^4.0.1",
     "msgflo": "^0.11.0"
   }
 }