Update dependency puma to v3.12.6 [SECURITY]

Author: renovate-bot

Gemfile.lock

@@ -90,7 +90,7 @@ GEM
     nio4r (2.3.1)
     nokogiri (1.8.4)
       mini_portile2 (~> 2.3.0)
-    puma (3.12.0)
+    puma (3.12.6)
     rack (2.0.5)
     rack-attack (5.4.0)
       rack (>= 1.0, < 3)