Bump rack from 1.1.2 to 1.1.6

Bumps [rack](https://github.com/rack/rack) from 1.1.2 to 1.1.6.
- [Release notes](https://github.com/rack/rack/releases)
- [Changelog](https://github.com/rack/rack/blob/master/CHANGELOG.md)
- [Commits](rack/rack@1.1.2...1.1.6)

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

Author: dependabot[bot]

Gemfile.lock

@@ -40,7 +40,7 @@ GEM
     net-ldap (0.3.1)
     nokogiri (1.5.0)
     pg (0.14.0)
-    rack (1.1.2)
+    rack (1.1.6)
     rack-test (0.6.1)
       rack (>= 1.0)
     rails (2.3.14)