Bump puma from 3.10.0 to 4.3.8

Bumps [puma](https://github.com/puma/puma) from 3.10.0 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@v3.10.0...v4.3.8)

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

Gemfile

@@ -12,7 +12,7 @@ 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', '~> 4.3'
 # Use SCSS for stylesheets
 gem 'sass-rails', '~> 5.0'
 # Use Uglifier as compressor for JavaScript assets

Gemfile.lock

@@ -99,7 +99,8 @@ GEM
       mini_portile2 (~> 2.3.0)
     pg (0.21.0)
     public_suffix (3.0.0)
-    puma (3.10.0)
+    puma (4.3.8)
+      nio4r (~> 2.0)
     rack (2.0.3)
     rack-test (0.7.0)
       rack (>= 1.0, < 3)
@@ -191,7 +192,7 @@ DEPENDENCIES
   listen (>= 3.0.5, < 3.2)
   materialize-sass
   pg (~> 0.18)
-  puma (~> 3.7)
+  puma (~> 4.3)
   rails (~> 5.1.4)
   sass-rails (~> 5.0)
   selenium-webdriver