Update mocha requirement from ^2.3.4 to ^5.2.0

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](https://github.com/mochajs/mocha/commits/v5.2.0)

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

package.json

@@ -31,6 +31,6 @@
     "eslint": "^5.9.0",
     "eslint-config-hexo": "^3.0.0",
     "istanbul": "^0.4.1",
-    "mocha": "^2.3.4"
+    "mocha": "^5.2.0"
   }
 }