Fix test suite by running Puma's master branch

The current release is incompatible with Capybara when also using Rackup
1.0.1, causing all feature specs to fail. The fix has been merged into
puma's `master` branch, so we can temporarily run that in CI.

Gemfile

@@ -21,6 +21,7 @@ gem 'pg', '~> 1.0', require: false if dbs.match?(/all|postgres/)
 gem 'fast_sqlite', require: false if dbs.match?(/all|sqlite/)
 gem 'sqlite3', '~> 1.4', require: false if dbs.match?(/all|sqlite/)
 
+
 gem 'database_cleaner', '~> 2.0', require: false
 gem 'rspec-activemodel-mocks', '~> 1.1', require: false
 gem 'rspec-rails', '~> 6.0.3', require: false
@@ -30,7 +31,12 @@ gem 'simplecov-cobertura', require: false
 gem 'rack', '< 3', require: false
 gem 'rake', require: false, groups: [:lint, :release]
 gem 'rails-controller-testing', require: false
-gem 'puma', '< 7', require: false
+
+# Temporarily use the master branch of puma until the next release
+# The current release, 6.4.3, has bug that causes Capybara to crash.
+# See https://github.com/puma/puma/pull/3532
+gem 'puma', github: 'puma/puma', branch: 'master', require: false
+
 gem 'i18n-tasks', '~> 0.9', require: false
 gem 'rspec_junit_formatter', require: false
 gem 'yard', require: false