chore(package): update mocha to version 6.2.2

Closes #21

Author: greenkeeper[bot]

package.json

@@ -32,6 +32,6 @@
   },
   "devDependencies": {
     "chai": "^4.0.2",
-    "mocha": "^3.3.0"
+    "mocha": "^6.2.2"
   }
 }