chore(package): update mqtt to version 4.0.0

Closes #33

Author: greenkeeper[bot]

package.json

@@ -18,7 +18,7 @@
     "coffee-script": "^1.8.0",
     "mocha": "^5.0.0",
     "msgflo": "^0.10.0",
-    "mqtt": "^2.15.0"
+    "mqtt": "^4.0.0"
   },
   "keywords": [],
   "bin": {},