Reverts unwanted change.

git-svn-id: http://svn.redmine.org/redmine/trunk@17537 e93f8b46-1217-0410-a6f0-8f06a7374b81

Gemfile

@@ -89,7 +89,7 @@ group :test do
   gem "mocha"
   gem "simplecov", "~> 0.14.1", :require => false
   # For running system tests
-  #gem 'puma', '~> 3.7'
+  gem 'puma', '~> 3.7'
   gem "capybara", '~> 2.13'
   gem "selenium-webdriver"
 end