Bump rack from 1.5.2 to 1.5.5

Bumps [rack](https://github.com/rack/rack) from 1.5.2 to 1.5.5.
- [Release notes](https://github.com/rack/rack/releases)
- [Changelog](https://github.com/rack/rack/blob/master/CHANGELOG.md)
- [Commits](https://github.com/rack/rack/compare/1.5.2...1.5.5)

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

Gemfile.lock

@@ -220,7 +220,7 @@ GEM
       rack (>= 1.1, < 2.0)
     quiet_assets (1.0.3)
       railties (>= 3.1, < 5.0)
-    rack (1.5.2)
+    rack (1.5.5)
     rack-protection (1.5.3)
       rack
     rack-test (0.6.2)