Bump puma from 3.12.6 to 4.3.8

Bumps [puma](https://github.com/puma/puma) from 3.12.6 to 4.3.8.
- [Release notes](https://github.com/puma/puma/releases)
- [Changelog](https://github.com/puma/puma/blob/master/History.md)
- [Commits](puma/puma@v3.12.6...v4.3.8)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

Gemfile

@@ -12,7 +12,7 @@ gem 'rails', '~> 5.1.3'
 # Use mysql as the database for Active Record
 gem 'mysql2', '>= 0.3.18', '< 0.5'
 # Use Puma as the app server
-gem 'puma', '~> 3.12'
+gem 'puma', '~> 4.3'
 # Use SCSS for stylesheets
 gem 'sass-rails', '~> 5.0'
 # Use Uglifier as compressor for JavaScript assets

Gemfile.lock

@@ -126,7 +126,8 @@ GEM
       racc (~> 1.4)
     orm_adapter (0.5.0)
     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-pjax (1.1.0)
@@ -249,7 +250,7 @@ DEPENDENCIES
   jbuilder (~> 2.5)
   listen (>= 3.0.5, < 3.2)
   mysql2 (>= 0.3.18, < 0.5)
-  puma (~> 3.12)
+  puma (~> 4.3)
   rails (~> 5.1.3)
   rails_admin (~> 1.4)
   sass-rails (~> 5.0)