chore(package): update mqtt to version 4.1.0

Closes #42

Author: greenkeeper[bot]

package.json

@@ -22,7 +22,7 @@
     "chai": "^4.1.0",
     "coffee-script": "^1.8.0",
     "mocha": "^5.0.0",
-    "mqtt": "^2.13.0",
+    "mqtt": "^4.1.0",
     "msgflo": "^0.11.0"
   },
   "keywords": [],