chore(package): update mocha to version 6.2.1

Closes #12

Author: greenkeeper[bot]

package.json

@@ -67,7 +67,7 @@
     "eslint-plugin-standard": "^3.0.1",
     "husky": "^0.14.3",
     "koaton-exporter": "^1.2.1",
-    "mocha": "^4.0.1",
+    "mocha": "^6.2.1",
     "npm-run-all": "^4.1.1",
     "nyc": "11.4.1",
     "ps-tree": "^1.1.0"