Update dependency puma to v3.12.4 [SECURITY]

Author: renovate-bot

Gemfile.lock

@@ -85,7 +85,7 @@ GEM
     nokogiri (1.10.7)
       mini_portile2 (~> 2.4.0)
     pg (0.21.0)
-    puma (3.12.2)
+    puma (3.12.6)
     rack (2.0.8)
     rack-proxy (0.6.5)
       rack