Bump puma from 3.12.4 to 3.12.6

Bumps [puma](https://github.com/puma/puma) from 3.12.4 to 3.12.6.
- [Release notes](https://github.com/puma/puma/releases)
- [Changelog](https://github.com/puma/puma/blob/master/History.md)
- [Commits](https://github.com/puma/puma/commits)

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

Gemfile.lock

@@ -157,7 +157,7 @@ GEM
     popper_js (1.14.5)
     powerpack (0.1.1)
     public_suffix (3.0.2)
-    puma (3.12.4)
+    puma (3.12.6)
     rack (2.0.8)
     rack-canonical-host (0.2.3)
       addressable (> 0, < 3)