build(deps): bump rack from 1.6.11 to 1.6.12 in /tools/rb/nutztest

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] <[email protected]>

Author: dependabot[bot]

tools/rb/nutztest/Gemfile.lock

@@ -6,7 +6,7 @@ GEM
     eventmachine (1.2.0.1)
     eventmachine (1.2.0.1-x86-mingw32)
     multi_json (1.12.1)
-    rack (1.6.11)
+    rack (1.6.12)
     rack-protection (1.5.5)
       rack
     rack-test (0.6.3)