Bump rack from 1.6.11 to 1.6.12 in /website (#8524)

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](https://github.com/rack/rack/compare/1.6.11...1.6.12)

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

website/Gemfile.lock

@@ -112,7 +112,7 @@ GEM
       tilt (>= 1.4.1, < 3)
     padrino-support (0.12.9)
       activesupport (>= 3.1)
-    rack (1.6.11)
+    rack (1.6.12)
     rack-livereload (0.3.17)
       rack
     rack-test (1.1.0)