Bump rack from 1.6.4 to 1.6.13

Bumps [rack](https://github.com/rack/rack) from 1.6.4 to 1.6.13.
- [Release notes](https://github.com/rack/rack/releases)
- [Changelog](https://github.com/rack/rack/blob/master/CHANGELOG.md)
- [Commits](rack/rack@1.6.4...1.6.13)

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

Gemfile.lock

@@ -107,7 +107,7 @@ GEM
     puma (3.5.2)
     quiet_assets (1.0.3)
       railties (>= 3.1, < 5.0)
-    rack (1.6.4)
+    rack (1.6.13)
     rack-test (0.6.3)
       rack (>= 1.0)
     rails (4.2.6)