Update puma to version 4.3.8

Gemfile

@@ -13,7 +13,7 @@ end
 gem 'rails', '~> 5.2'
 
 # Use Puma as the app server
-gem 'puma', '~> 3.0'
+gem 'puma', '~> 4.3'
 
 # respond_to methods have been extracted to the responders gem
 # http://edgeguides.rubyonrails.org/upgrading_ruby_on_rails.html#responders

Gemfile.lock

@@ -400,7 +400,8 @@ GEM
       method_source (~> 0.8.1)
       slop (~> 3.4)
     public_suffix (4.0.6)
-    puma (3.12.6)
+    puma (4.3.8)
+      nio4r (~> 2.0)
     racc (1.5.2)
     rack (2.2.3)
     rack-openid (1.3.1)
@@ -701,7 +702,7 @@ DEPENDENCIES
   piwik_analytics (~> 1.0.1)
   prawn-qrcode
   prawn-rails
-  puma (~> 3.0)
+  puma (~> 4.3)
   rails (~> 5.2)
   rails-assets-bootstrap-markdown!
   rails-assets-bootstrap-select!