Update mocha requirement from ^5.2.0 to ^6.0.2

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@v5.2.0...v6.0.2)

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

package.json

@@ -41,7 +41,7 @@
     "hexo": "^3.7.1",
     "hexo-fs": "^1.0.0",
     "istanbul": "^0.4.1",
-    "mocha": "^5.2.0",
+    "mocha": "^6.0.2",
     "sinon": "^7.0.0",
     "supertest": "^1.2.0",
     "supertest-promised": "^1.0.0",