chore(deps-dev): bump koa from 2.7.0 to 2.8.1

Bumps [koa](https://github.com/koajs/koa) from 2.7.0 to 2.8.1.
- [Release notes](https://github.com/koajs/koa/releases)
- [Changelog](https://github.com/koajs/koa/blob/master/History.md)
- [Commits](koajs/koa@2.7.0...2.8.1)

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

package.json

@@ -39,7 +39,7 @@
     "elint": "^2.0.0-alpha.1",
     "elint-preset-kw": "^1.8.1",
     "get-port": "^5.0.0",
-    "koa": "^2.5.2",
+    "koa": "^2.8.1",
     "koa-router": "^7.4.0",
     "mocha": "^6.2.0",
     "nyc": "^14.1.1",