Bump eslint from 4.19.1 to 5.9.0 (#822)

Bumps [eslint](https://github.com/eslint/eslint) from 4.19.1 to 5.9.0.
- [Release notes](https://github.com/eslint/eslint/releases)
- [Changelog](https://github.com/eslint/eslint/blob/master/CHANGELOG.md)
- [Commits](eslint/eslint@v4.19.1...v5.9.0)

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

package.json

@@ -26,7 +26,7 @@
   },
   "devDependencies": {
     "del": "^3.0.0",
-    "eslint": "^4.3.0",
+    "eslint": "^5.9.0",
     "eslint-config-hexo": "^2.0.0",
     "gulp": "^3.9.1",
     "gulp-cheerio": "^0.6.2",