Update eslint requirement from ^4.19.1 to ^5.9.0

Updates the requirements on [eslint](https://github.com/eslint/eslint) to permit the latest version.
- [Release notes](https://github.com/eslint/eslint/releases)
- [Changelog](https://github.com/eslint/eslint/blob/master/CHANGELOG.md)
- [Commits](https://github.com/eslint/eslint/commits/v5.9.0)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot-support

package.json

@@ -45,7 +45,7 @@
   "devDependencies": {
     "chai": "^3.5.0",
     "chai-as-promised": "^5.3.0",
-    "eslint": "^4.19.1",
+    "eslint": "^5.9.0",
     "eslint-config-hexo": "^3.0.0",
     "hexo-renderer-marked": "^0.2.10",
     "istanbul": "^0.4.3",