Bump puma from 3.12.2 to 4.3.8

Bumps [puma](https://github.com/puma/puma) from 3.12.2 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@v3.12.2...v4.3.8)

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

Author: dependabot[bot]

Gemfile

@@ -8,7 +8,7 @@ gem 'rails', '~> 5.2.3'
 # For database
 gem 'pg'
 # 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

@@ -115,7 +115,8 @@ GEM
       ast (~> 2.4.0)
     pg (1.1.4)
     public_suffix (4.0.1)
-    puma (3.12.2)
+    puma (4.3.8)
+      nio4r (~> 2.0)
     racc (1.5.2)
     rack (2.0.8)
     rack-test (1.1.0)
@@ -220,7 +221,7 @@ DEPENDENCIES
   jbuilder (~> 2.5)
   listen (>= 3.0.5, < 3.2)
   pg
-  puma (~> 3.12)
+  puma (~> 4.3)
   rails (~> 5.2.3)
   rubocop
   sass-rails (~> 5.0)