Bump rack from 1.6.11 to 1.6.12 in /gears/carto_gears_api

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>

gears/carto_gears_api/Gemfile.lock

@@ -68,7 +68,7 @@ GEM
     nokogiri (1.10.5)
       mini_portile2 (~> 2.4.0)
     pg (0.20.0)
-    rack (1.6.11)
+    rack (1.6.12)
     rack-test (0.6.3)
       rack (>= 1.0)
     rails (4.2.11)