Bump eslint from 3.19.0 to 6.6.0

Bumps [eslint](https://github.com/eslint/eslint) from 3.19.0 to 6.6.0.
- [Release notes](https://github.com/eslint/eslint/releases)
- [Changelog](https://github.com/eslint/eslint/blob/master/CHANGELOG.md)
- [Commits](eslint/eslint@v3.19.0...v6.6.0)

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

package.json

@@ -22,7 +22,7 @@
   "homepage": "https://github.com/JacksonTian/httpx",
   "devDependencies": {
     "coveralls": "^2.11.15",
-    "eslint": "^3.13.1",
+    "eslint": "^6.6.0",
     "mocha": "^4",
     "nyc": "^12.0.2"
   },