chore(package): update mocha to version 6.2.0

Closes #9

package.json

@@ -37,7 +37,7 @@
     "assume": "~2.0.0",
     "eventstub": "1.0.x",
     "istanbul": "~0.4.5",
-    "mocha": "~3.5.0",
+    "mocha": "~6.2.0",
     "pre-commit": "~1.2.0"
   }
 }