chore(deps): upgrade puma to 4.3.8

Gemfile

@@ -1,7 +1,7 @@
 source 'https://rubygems.org'
 gem 'rails', '= 5.2.5'
 gem 'mysql2'
-gem 'puma'
+gem 'puma', '~> 4'
 gem 'turbolinks', '~> 5'
 gem 'haml'
 gem 'nifty-utils'

Gemfile.lock

@@ -152,7 +152,7 @@ GEM
     nokogiri (1.11.2)
       mini_portile2 (~> 2.5.0)
       racc (~> 1.4)
-    puma (4.3.5)
+    puma (4.3.8)
       nio4r (~> 2.0)
     racc (1.5.2)
     rack (2.2.3)
@@ -279,7 +279,7 @@ DEPENDENCIES
   nifty-utils
   nilify_blanks
   nio4r
-  puma
+  puma (~> 4)
   rails (= 5.2.5)
   rspec
   rspec-rails