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] <[email protected]>

Author: dependabot[bot]

Gemfile.lock

@@ -19,7 +19,7 @@ GEM
     mongo (1.12.5)
       bson (= 1.12.5)
     netrc (0.11.0)
-    rack (1.6.11)
+    rack (1.6.12)
     rack-protection (1.5.5)
       rack
     rest-client (2.0.0)