chore(package): update mqtt to version 4.0.1

Closes #58

Author: greenkeeper[bot]

package.json

@@ -29,7 +29,7 @@
     "grunt-mocha-test": "^0.13.2",
     "grunt-shell-spawn": "^0.3.1",
     "mocha": "^5.0.0",
-    "mqtt": "^2.7.1"
+    "mqtt": "^4.0.1"
   },
   "keywords": [],
   "bin": {