chore(package): update mqtt to version 2.9.3

Closes #11

Author: greenkeeper[bot]

package.json

@@ -30,7 +30,7 @@
     "coffee-script": "^1.10.0",
     "mocha": "^3.2.0",
     "amqplib": "^0.5.1",
-    "mqtt": "~2.7.2",
+    "mqtt": "~2.9.3",
     "msgflo": "^0.10.19"
   }
 }