Fix security issue with puma (#22)

Author: atomaka

spec/rails/Gemfile

@@ -4,7 +4,7 @@ git_source(:github) { |repo| "https://github.com/#{repo}.git" }
 # Bundle edge Rails instead: gem 'rails', github: 'rails/rails'
 gem "rails", "~> 6.0.0"
 # Use Puma as the app server
-gem "puma", "~> 3.11"
+gem "puma", ">= 4.3.8"
 # Build JSON APIs with ease. Read more: https://github.com/rails/jbuilder
 # gem 'jbuilder', '~> 2.7'
 # Use Active Model has_secure_password