Bump rack from 1.6.12 to 1.6.13 (brentley#33)

Bumps [rack](https://github.com/rack/rack) from 1.6.12 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@1.6.12...1.6.13)

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

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

Gemfile.lock

@@ -79,7 +79,7 @@ GEM
       mini_portile2 (~> 2.5.0)
       racc (~> 1.4)
     racc (1.5.2)
-    rack (1.6.12)
+    rack (1.6.13)
     rack-test (0.6.3)
       rack (>= 1.0)
     rails (4.2.10)