chore(package): update mocha to version 6.0.2

Closes #9

package.json

@@ -29,7 +29,7 @@
   "devDependencies": {
     "assume": "~2.0.0",
     "istanbul": "~0.4.5",
-    "mocha": "~3.5.0",
+    "mocha": "~6.0.2",
     "pre-commit": "~1.2.0"
   }
 }