Bump rack from 1.6.11 to 1.6.13

Bumps [rack](https://github.com/rack/rack) from 1.6.11 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@1.6.11...1.6.13)

---
updated-dependencies:
- dependency-name: rack
  dependency-type: indirect
...

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

Author: dependabot[bot]

Gemfile.lock

@@ -207,7 +207,7 @@ GEM
       pry (>= 0.10.4)
     quiet_assets (1.1.0)
       railties (>= 3.1, < 5.0)
-    rack (1.6.11)
+    rack (1.6.13)
     rack-cors (1.0.3)
     rack-test (0.6.3)
       rack (>= 1.0)