Update puma to v4.3.1

This is a security release to patch CVE-2019-16770:
https://github.com/puma/puma/blob/master/History.md

Gemfile.lock

@@ -748,7 +748,7 @@ GEM
     pry-rails (0.3.6)
       pry (>= 0.10.4)
     public_suffix (3.1.1)
-    puma (4.3.0)
+    puma (4.3.1)
       nio4r (~> 2.0)
     puma_worker_killer (0.1.1)
       get_process_mem (~> 0.2)