Update mocha requirement from ^2.3.3 to ^6.2.0

Updates the requirements on [mocha](https://github.com/mochajs/mocha) to permit the latest version.
- [Release notes](https://github.com/mochajs/mocha/releases)
- [Changelog](https://github.com/mochajs/mocha/blob/master/CHANGELOG.md)
- [Commits](mochajs/mocha@v2.3.3...v6.2.0)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

package.json

@@ -23,7 +23,7 @@
     "eslint-config-hexo": "^3.0.0",
     "hexo": "^3.9.0",
     "istanbul": "^0.4.0",
-    "mocha": "^2.3.3"
+    "mocha": "^6.2.0"
   },
   "engines": {
     "node": ">=8.6.0"