chore(package): update mocha to version 6.1.1

Closes #15

Author: greenkeeper[bot]

package.json

@@ -41,6 +41,6 @@
     "concat-stream": "^1.4.6",
     "event-stream": "^3.3.2",
     "gulp": "^3.0.0",
-    "mocha": "^3.5.3"
+    "mocha": "^6.1.1"
   }
 }