Bump rack from 1.4.1 to 1.6.13 in /70-3rd-March-2020/keynote

Bumps [rack](https://github.com/rack/rack) from 1.4.1 to 1.6.13.
- [Release notes](https://github.com/rack/rack/releases)
- [Changelog](https://github.com/rack/rack/blob/main/CHANGELOG.md)
- [Commits](rack/rack@1.4.1...1.6.13)

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

Signed-off-by: dependabot[bot] <[email protected]>

70-3rd-March-2020/keynote/Gemfile.lock

@@ -1,7 +1,7 @@
 GEM
   remote: http://rubygems.org/
   specs:
-    rack (1.4.1)
+    rack (1.6.13)
     rack-protection (1.2.0)
       rack
     shotgun (0.9)