build: bump koa-static from 4.0.3 to 5.0.0 (#260)

Bumps [koa-static](https://github.com/koajs/static) from 4.0.3 to 5.0.0.
- [Release notes](https://github.com/koajs/static/releases)
- [Changelog](https://github.com/koajs/static/blob/master/History.md)
- [Commits](koajs/static@4.0.3...5.0.0)

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

package.json

@@ -42,7 +42,7 @@
     "koa": "2.5.1",
     "koa-favicon": "2.0.1",
     "koa-rewrite": "3.0.1",
-    "koa-static": "4.0.3",
+    "koa-static": "5.0.0",
     "mkdirp": "0.5.1",
     "mocha": "5.2.0",
     "moment": "2.22.2",