test: bump puma in rails 5.2 test app to fix security warning (#377)

Author: pkuczynski

spec/app/rails_5.2/Gemfile

@@ -5,7 +5,7 @@ gem 'rails', '~> 5.2.2'
 # Use sqlite3 as the database for Active Record
 gem 'sqlite3'
 # Use Puma as the app server
-gem 'puma', '~> 4.3.8'
+gem 'puma', '~> 5.6.9'
 # Use SCSS for stylesheets
 gem 'sass-rails', '~> 5.0'
 # Use Uglifier as compressor for JavaScript assets