Bump puma from 3.11.2 to 3.12.6 (heroku#68)

Bumps [puma](https://github.com/puma/puma) from 3.11.2 to 3.12.6.
- [Release notes](https://github.com/puma/puma/releases)
- [Changelog](https://github.com/puma/puma/blob/master/History.md)
- [Commits](puma/puma@v3.11.2...v3.12.6)

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

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

Gemfile.lock

@@ -92,7 +92,7 @@ GEM
     nokogiri (1.10.5)
       mini_portile2 (~> 2.4.0)
     pg (0.19.0)
-    puma (3.11.2)
+    puma (3.12.6)
     rack (2.2.3)
     rack-test (1.1.0)
       rack (>= 1.0, < 3)