Bump rack from 1.6.11 to 1.6.12

Bumps [rack](https://github.com/rack/rack) from 1.6.11 to 1.6.12.
- [Release notes](https://github.com/rack/rack/releases)
- [Changelog](https://github.com/rack/rack/blob/master/CHANGELOG.md)
- [Commits](rack/rack@1.6.11...1.6.12)

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

Author: dependabot[bot]

Gemfile.lock

@@ -81,7 +81,7 @@ GEM
       tilt (>= 1.4.1, < 3)
     padrino-support (0.12.9)
       activesupport (>= 3.1)
-    rack (1.6.11)
+    rack (1.6.12)
     rack-test (0.6.3)
       rack (>= 1.0)
     rb-fsevent (0.10.3)