fix(package): update mqtt to version 4.1.0

Closes #42

Author: greenkeeper[bot]

package.json

@@ -23,7 +23,7 @@
     "carrier": "~0.3.0",
     "line-reader": "~0.4.0",
     "vr": "JaniL/vr#5c66020738fd6f229e9f9a51bd8649b98b1bc859",
-    "mqtt": "^2.15.0"
+    "mqtt": "^4.1.0"
   },
   "devDependencies": {
     "grunt-contrib-watch": "~1.0.0",