Bump puma from 3.12.2 to 4.3.1

Bumps [puma](https://github.com/puma/puma) from 3.12.2 to 4.3.1.
- [Release notes](https://github.com/puma/puma/releases)
- [Changelog](https://github.com/puma/puma/blob/master/History.md)
- [Commits](puma/puma@v3.12.2...v4.3.1)

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

Author: dependabot-preview[bot]

Gemfile

@@ -6,7 +6,7 @@ gem 'rails', '5.1.7'
 # Use postgresql as the database for Active Record
 gem 'pg', '~> 0.18'
 # Use Puma as the app server
-gem 'puma', '~> 3.12'
+gem 'puma', '~> 4.3'
 # use Devise Token Auth for token based authentication
 gem 'devise_token_auth'
 gem 'omniauth'

Gemfile.lock

@@ -110,7 +110,8 @@ GEM
     pry-stack_explorer (0.4.9.3)
       binding_of_caller (>= 0.7)
       pry (>= 0.9.11)
-    puma (3.12.2)
+    puma (4.3.1)
+      nio4r (~> 2.0)
     rack (2.1.2)
     rack-attack (5.0.1)
       rack
@@ -223,7 +224,7 @@ DEPENDENCIES
   pry-byebug
   pry-rails
   pry-stack_explorer
-  puma (~> 3.12)
+  puma (~> 4.3)
   rack-attack
   rack-cors
   rails (= 5.1.7)