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.
- [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[bot] <[email protected]>

Author: dependabot[bot]

Gemfile.lock

@@ -62,7 +62,7 @@ GEM
     nio4r (2.0.0)
     nokogiri (1.7.2)
       mini_portile2 (~> 2.1.0)
-    rack (2.0.3)
+    rack (2.2.3)
     rack-test (0.6.3)
       rack (>= 1.0)
     rails (5.1.1)