Update dependency puma to v4 [SECURITY]

Author: renovate-bot

Gemfile

@@ -5,7 +5,7 @@ gem 'rails', '~> 5.1.4'
 # Use postgresql as the database for Active Record
 gem 'pg', '~> 0.18'
 # Use Puma as the app server
-gem 'puma', '~> 3.7'
+gem 'puma', '~> 4.0'
 # Use SCSS for stylesheets
 gem 'sass-rails', '~> 5.0'
 # Use Uglifier as compressor for JavaScript assets

Gemfile.lock

@@ -85,7 +85,8 @@ GEM
     nokogiri (1.10.7)
       mini_portile2 (~> 2.4.0)
     pg (0.21.0)
-    puma (3.12.2)
+    puma (4.3.10)
+      nio4r (~> 2.0)
     rack (2.0.8)
     rack-proxy (0.6.5)
       rack
@@ -178,7 +179,7 @@ DEPENDENCIES
   jbuilder (~> 2.5)
   listen (>= 3.0.5, < 3.2)
   pg (~> 0.18)
-  puma (~> 3.7)
+  puma (~> 4.0)
   rails (~> 5.1.4)
   sass-rails (~> 5.0)
   spring