Update chai requirement from ^3.4.1 to ^4.2.0

Updates the requirements on [chai](https://github.com/chaijs/chai) to permit the latest version.
- [Release notes](https://github.com/chaijs/chai/releases)
- [Changelog](https://github.com/chaijs/chai/blob/master/History.md)
- [Commits](https://github.com/chaijs/chai/commits/4.2.0)

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

package.json

@@ -18,7 +18,7 @@
   "author": "Tommy Chen <tommy351@gmail.com> (http://zespia.tw)",
   "license": "MIT",
   "devDependencies": {
-    "chai": "^3.4.1",
+    "chai": "^4.2.0",
     "eslint": "^6.0.1",
     "eslint-config-hexo": "^3.0.0",
     "hexo": "^3.9.0",