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)

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

Author: dependabot[bot]

Gemfile.lock

@@ -106,7 +106,7 @@ GEM
     padrino-support (0.12.8.1)
       activesupport (>= 3.1)
     ptools (1.3.5)
-    rack (1.6.11)
+    rack (1.6.13)
     rack-livereload (0.3.16)
       rack
     rack-test (0.8.2)