Update mocha requirement from ^5.2.0 to ^6.0.2 (#130)

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

@@ -34,7 +34,7 @@
     "eslint": "^5.6.1",
     "eslint-config-hexo": "^3.0.0",
     "istanbul": "^0.4.3",
-    "mocha": "^5.2.0"
+    "mocha": "^6.0.2"
   },
   "dependencies": {
     "bluebird": "^3.5.0",