[Security] Bump rack from 2.0.3 to 2.2.3

Bumps [rack](https://github.com/rack/rack) from 2.0.3 to 2.2.3. **This update includes security fixes.**
- [Release notes](https://github.com/rack/rack/releases)
- [Changelog](https://github.com/rack/rack/blob/master/CHANGELOG.md)
- [Commits](rack/rack@2.0.3...2.2.3)

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

Gemfile.lock

@@ -98,7 +98,7 @@ GEM
       mini_portile2 (~> 2.2.0)
     pg (0.21.0)
     puma (3.10.0)
-    rack (2.0.3)
+    rack (2.2.3)
     rack-test (0.6.3)
       rack (>= 1.0)
     rails (5.1.3)