Update chai requirement from ^3.5.0 to ^4.2.0 (#3)

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

@@ -23,7 +23,7 @@
     "object-assign": "^4.0.1"
   },
   "devDependencies": {
-    "chai": "^3.5.0",
+    "chai": "^4.2.0",
     "eslint": "^2.13.1",
     "eslint-config-hexo": "^1.0.2",
     "istanbul": "^0.4.2",