[Security] Bump puma from 3.12.0 to 4.3.8

Bumps [puma](https://github.com/puma/puma) from 3.12.0 to 4.3.8. **This update includes security fixes.**
- [Release notes](https://github.com/puma/puma/releases)
- [Changelog](https://github.com/puma/puma/blob/master/History.md)
- [Commits](puma/puma@v3.12.0...v4.3.8)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Gemfile

@@ -10,7 +10,7 @@ gem 'rails', '~> 5.2.3'
 # Use postgresql as the database for Active Record
 gem 'pg', '>= 0.18', '< 2.0'
 # Use Puma as the app server
-gem 'puma', '~> 3.7'
+gem 'puma', '~> 4.3'
 # Use SCSS for stylesheets
 gem 'sass-rails', '~> 5.0'
 # Use Uglifier as compressor for JavaScript assets

Gemfile.lock

@@ -127,7 +127,9 @@ GEM
     mime-types (3.2.2)
       mime-types-data (~> 3.2015)
     mime-types-data (3.2018.0812)
-    mimemagic (0.3.3)
+    mimemagic (0.3.10)
+      nokogiri (~> 1)
+      rake
     mini_mime (1.0.1)
     mini_portile2 (2.4.0)
     minitest (5.11.3)
@@ -150,7 +152,8 @@ GEM
     popper_js (1.14.5)
     powerpack (0.1.2)
     public_suffix (3.0.3)
-    puma (3.12.0)
+    puma (4.3.8)
+      nio4r (~> 2.0)
     pundit (2.0.0)
       activesupport (>= 3.0.0)
     rack (2.0.7)
@@ -325,7 +328,7 @@ DEPENDENCIES
   jbuilder (~> 2.8)
   jquery-rails
   pg (>= 0.18, < 2.0)
-  puma (~> 3.7)
+  puma (~> 4.3)
   pundit
   rails (~> 5.2.3)
   rails-controller-testing